muy fácil‘very easy’muy difícil‘very difficult’Facilidad comprensión‘Ease of comprehension’

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Make It Big!
The Effect of Font Size and Line Spacingon Online Readability

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Martin Pielot
Telefonica Research
Placa Ernest Lluch i Marti, 5Barcelona, 08019, Spainmartin.pielot@telefonica.com
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Luz Rello
Human-Computer Interaction
InstituteCarnegie Mellon University
5000 Forbes Avenue
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Pittsburgh, PA 15213, USAluzrello@cs.cmu.edu

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Mari-Carmen Marcos
DigiDoc & Web Research
Group
Universitat Pompeu Fabra
Barcelona, Spain
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mcarmen.marcos@upf.edu
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ABSTRACTWe report from an eye-tracking experiment with 104
participants who performed reading tasks on the mostpopular text-heavy website of the Web: Wikipedia. Using a hybrid-measures design, we compared objective
and subjective readability and comprehension of the articles for font sizes ranging from 10 to 26 points, andline spacings ranging from 0.8 to 1.8 (font: Arial ). Ourfindings provide evidence that readability, measured via
mean fixation duration, increased significantly with fontsize. Further, comprehension questions had significantly
more correct responses for font sizes 18 and 26. Forline spacing, we found marginal e↵ects, suggesting thatthe two tested extremes (0.8 and 1.8) impair readability.
These findings provide evidence that text-heavy websites
should use fonts of size 18 or larger and use default line
spacing when the goal is to make a web page easy to readand comprehend. Our results significantly di↵er fromprevious recommendations, presumably, because this is
the first work to cover font sizes beyond 14 points.
Author Keywords

Readability; comprehension; font size; line spacing;
online reading; text presentation; eye-tracking;
Wikipedia.
ACM Classification Keywords

H.5.0 Information Interfaces and Presentation: General;
H.5.2 Information Interfaces and Presentation: User Interfaces—Screen design, style guides.
INTRODUCTION

While it may seem a little old-fashioned, reading is still
one of the primary ways to interact with computing devices. And as more and more content and services move
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Copyright c 2016 ACM ISBN/978-1-4503-3362-7/16/05...$15.00.
http://dx.doi.org/10.1145/2858036.2858204

Figure 1. Example of a Wikipedia article used in theexperiment.

online, reading increasingly takes places on screens andin web browsers. Improving readability of text in the
Web is one of the most simple and e↵ective ways to improve usability and ease access to information – also forpeople with special needs, such as elderly people [14], or
people with print disabilities [26], such as people withlow vision [18] or dyslexia [23, 29].
In this work, readability refers to the ease with which areader can read and understand a written text. It only
refers to the properties of the presentation of the text,
not the content of the text itself. One of the crucial
factors for readability in this context is font size [21, 24]
together with line spacing [25].
Designers who try to find the optimal font size and line
spacing for their web pages face a myriad of conflicting
recommendations. Jakob Nielsen, one of the experts in
web usability, stresses to allow users to control the size

of the font [21].1 However, as many users will not adjust
the text display settings, he suggests to use 10 points
as minimum font size. According to a survey on typographic design patterns and best practices [20], in 2009
the most used font size was 13 points and the most used
line spacing was 148% (line spacing of 1.23) of the fontsize. In a replication of the study in 2013, the font sizes
14 and 16 points were most popular [13]. Yet, these numbers represent best-practices, which are not necessarily
backed up by scientific studies.
Research on font size has a long history in the HCI Community. Previous research [1, 2, 3, 4, 5, 8, 14] has extensively explored font sizes of 10, 12, and 14 points, andtypically found that the biggest tested font size leads tothe best results. However, this leaves a number of open
questions. First, it has largely ignored line spacing as
parameter [9]. Second, the studies mostly focus on readability, while comprehension, i.e., whether the reader actually understood the content of the text, is mostly notmeasured. Third, we did not find any general-population
study exploring font sizes above 16 points, even thoughprevious findings indicate bigger fonts lead to better results.
In this paper, we report from a user study that addresses
these open questions. With 104 participants, we conducted an experiment to determine the e↵ect of font sizeand line spacing on readability and comprehension of
texts on websites. Using a 17-inch monitor with integrated eye-tracker, the participants had to read web text
in a Firefox browser, displayed in its default sans-serif
font: Arial. We compared 6 font sizes (10, 12, 14, 18,
22, and 26 points) and 4 line spacings (0.8, 1.0, 1.4, and
1.8). The participants had to read articles from a popular text-heavy website: Wikipedia (see Figure 1).
Our main findings are:– Font size had significant and large e↵ects on readability and comprehension. Both aspects improved significantly with increasing font sizes until 18/22 points –far beyond the typical recommendations of 10/12/14
points.
– Line spacing had a small but significant e↵ect on comprehension, suggesting that too small or too large
spacings may impair comprehension.
– On the basis of our findings, we recommend to use 18
points font size and default line spacing if the goal is
to optimize readability and comprehension of web text
content.

RELATED WORK
Font Size

In his article on the Top 10 Mistakes in Web Design,2Jakob Nielsen argues that using too small font sizes is oneof the most frequent mistakes made in today’s website
design. Preferably, users should be allowed to adjust
the font size to their individual needs. Yet, Nielsen also
points out that users are typically too lazy to change
fonts when viewing websites. Consequently, to ensure
good readability, it is essential for websites to provide
appropriate defaults. Nielsen recommends to use fontsizes of at least 10 points or 12 points for elderly readers.
However, previous research come to di↵erent conclusions
about the ideal font size:
Bernard et al. [3] performed a study with 60 participants measuring reading time, preference, and errors
while reading the text out loud using eight di↵erent font
types and 10, 12 and 14 points. Fonts of 10 points wereread significantly more slowly than fonts of 12 points. In
a subsequent experiment, Bernard et al. [4] compared
the readability of two fonts –Arial and Times– and two
font sizes –10 and 12 points– in an experiment with 35
participants. The experiment used the same dependentmeasures. 10-point Arial typeface again was read slower
than the other conditions and the 12-point Arial typeface
was preferred to the other typefaces.
In order to understand the impact of age on reading,
Bernard et al. [2] studied the e↵ects of font type andsize on the legibility and reading time of online text byolder adults. They compared legibility, reading time,
and the participants’ preferences of texts displayed withsans-serif and serif fonts, and font sizes 12 and 14 points.
The 12-point serif fonts were read out loud significantly
slower than 14 serif and sans-serif fonts, and participantspreferred larger font sizes.
In the context of handheld computers, Darroch et al.
[14] investigated the e↵ect of font sizes ranging fromtwo to 16 points, measuring the reading speed (silent
reading), reading accuracy, and subjective views among
two groups, 12 old and 12 young readers. They did not
find any significant di↵erences neither between the agegroups, nor for the font sizes ranging from 6 to 16 points,
which may be due to the rather small sample size.
Banerjee et al. [1] performed a study with 40 participants, who had to read texts aloud using the font sizes
10, 12, and 14 points. Sitting at a distance of 60 to 70cm
from the monitor, the 14-point fonts lead to a significant
faster reading and was preferred over smaller font sizes.
Bhatia et al. [8] studied the e↵ect of, amongst otherfactors, font size on readability. A group of 180 undergraduate students had to take part in a text-reading experiment and indicate their preferences in a survey. The
font sizes that Bhatia et al. tested were 10, 12, and1

http://www.nngroup.com/articles/
let-users-control-font-size/2
http://www.nngroup.com/articles/
top-10-mistakes-web-design/, 2011,
2013.

last

visitedSep

07,

14 points. Unfortunately, the survey responses did notreveal any significant e↵ects.
The related work described until now approximated text
readability via preferences, reading time, and errors
made while reading the text aloud. However, these measures have drawbacks. Subjective readability may not
match objective readability. Fast reading times do notnecessarily indicate good readability. For example, participants may simply skim texts which are difficult toread. Reading aloud may introduce unsystematic variance through the extra reading activity, and is not ecologically valid for web reading.
As a remedy, Beymer and Russel [6] explored the useof objective measures of readability: they developed
WebGazeAnalyzer, a system to monitor reading performance with an eye tracker. This system allows, amongst
other things, to record the eyes’ fixations durations,which are an objective indicator of text readability [19,27, 31]. The longer the eyes fixate text parts at a time,
the higher the likelihood that the reader is encountering
difficulties in reading the text. Using this system in a
between-subjects design with 82 participants, Beymer et
al. [5] studied the e↵ect of the font sizes 10, 12, and 14
points on readability and comprehension scores. Whenusing 10 points font size, fixation durations resulted
significantly longer as compared to 14 points. They
also found significant di↵erences taking into account themother language of the participants, non-native English
subjects had significantly longer fixations.
In a similar setup, Rello et al. [29] studied the e↵ect offont size on the ability of people who were diagnosed withdyslexia to read texts. They found that larger font sizes
than the usual 10, 12, 14 points led to shorter fixation
durations.
The findings from previous work unanimously indicatethat the font sizes of 10 to 12 points, as suggested byNielsen and other sources, might be suboptimal, andthat font sizes that are larger than the traditional 10, 12,
14 points led to significant improvements in readability
and comprehension.
Dependencies of Font SizePrevious work indicates that font size is interdependent
with font type [3, 5]. Most of the previous work applies tothe two most common fonts used on screen and printed
texts, Arial and Times, respectively [12]. One of thereasons is that font size can result into di↵erent letter
sizes for di↵erent font types,3 so parts of the observede↵ects might be due to the actual size of the letters. In
consequence, research on the e↵ect of font size needs toconsider the font type, e.g., by at least making clear forwhich font type the findings are valid, or consider lettersof the same real size event if di↵erent in point size.
3
For the interested reader, see Figure 1 in Boyarski et al.
[10], who compared Times with Georgia and Verdana.

In addition, notice that if the column width is fixed, thenumber of character per column depends on the font size.
Line SpacingLine spacing refers to the vertical distance between thebaselines of two text lines. The concept is also know
as leading from the days of hand-typesetting and lineheight in CSS. The bigger the line spacing, the further
two sentences are apart vertically.
We found no specific guidelines for line spacing of web
texts. By default, browsers compute the line spacing relative to the font size. A spacing of 1.0 equates to 120%of the font size.4 In best-practice recommendations, thisspacing of 1.0 is often named as “generally the most readable and doesn’t require that you do anything special”.5
However, no studies are cited.
According to a review by Bix [9], the vast amount ofliterature indicates that the optimal amount of spacing
highly depends on other factors. Except for the generalrecommendation to avoid too little and too much spacing, no rules are given.
Paterson and Tinker [25] studied the e↵ect of line spacing in printed text when performing a reading test
(Chapman-Cook Speed) with 400 college students. They
found that bigger line spacings (1.2 and 1.4 compared to1.1) lead to faster readings. However the authors pointout that such results may depend on other factors suchas the font type and the column width, similarly to fontsize, as already mentioned.
Rello and Marcos [28] studied the e↵ect of line spacing(0.8, 1.0, 1.4, and 1.8) for reading raw text on the screen
using eye-tracking with 92 participants. They did not
find a significant e↵ect of line spacing on fixation duration.
What is Missing

In all presented previous studies, bigger fonts led to better results, either in terms of readability [5] or in terms ofpreference [2]. Thus, previous work indicates that bigger
font sizes will result into more readable websites. However, in the context of desktop computers, the biggest
font size studied was 14 points. Thus, previous work
does not answer whether this trend continues or flattens
out with increasing font sizes. About line spacing, noconclusive evidence has been reported.
METHODOLOGY

To study the e↵ect of font size and line spacing on readability and comprehension of websites, we conducted aneye-tracking experiment comparing font sizes from 10 to
26 points and line spacings from 0.8 to 1.8 times thestandard spacing. We had 104 participants that read six
Wikipedia entries related to animals with di↵erent font4
http://stackoverflow.com/questions/2262543/
css-line-height-guide5
http://webdesign.about.com/od/styleproperties/qt/css_
line_spacing.htm

6
Wikipedia is the seventh most popular website worldwide,
according to Alexa ranking: http://www.alexa.com/topsites(consulted Dec 18, 2015).
7
That is, 0.8 equals to 96% , 1.0 equals to 120%, 1.4 equals
to 168%, and 1.8 equals to 216% of the font size.

Facilidad comprensión‘Ease of comprehension’

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Participants

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Following our ●IRB requirements, we make a public announce and sent it to the schools
and universities of a city●●●
district. Potential participants ●●contactedusand came to●●
our lab at the Universitat Pompeu Fabra (UPF). Minors
were accompanied by a parent. Most of the participants
were students. Participation was voluntary.
104 volunteers● (61 ●female,
43 male)took
part in the●●●●
study. Their ages ranged from 14 to 54 (x̄● = 30.24, s =9.13) and they10all hador corrected-to-normal
vi12 normal
14
18
22
26
sion. Most of the participants
hadhighereducation.
ExFontFont Size
Size
cept from 5 participants, 7 participants were attending
school or high school and 92 participants were studying

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Figure 2. Comprehension perception scale rating.

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According to previous work [19, 27, 31], fixation duration is a valid and objective proxy for readability. The
rationale put forward by Just and Carpenter [19] is that
“readers make longer pauses at points where processing
loads are greater.” Rayner and Du↵y [27] write that
“There is now a fair amount of evidence to indicate thatsome of the variability [of fixation duration] is due to systematic di↵erences in the ease of processing the words
in the text.” Hence, if the mean duration of fixations

muy fácil
‘very easy’muy difícil‘very difficult’

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Fixation Duration: We used mean fixation duration
as objective approximation of readability. When reading a text, the eye does not move contiguously over the
text, but alternates saccades and visual fixations, thatis, jumps in short steps and rests on parts of the text.
Fixation duration denotes how long the eye rests still ona single place of the text.

Fixation
Fixation Duration
Duration Mean
Mean (ms)(ms)

For quantifying readability and comprehension, we used
the following dependent measures:

Subjective Perception Rating: In addition, we asked
the participants to provide their subjective perceptions.
For each of the six texts, the participants rated on two
five-point Likert scales, how easy it was for them to readand to understand the text. This defines the Subjective
Readability Rating and Subjective Comprehension Rating, respectively. Sauro and Dumas [30] showed that,given a sufficiently large sample, single-item scales are avery easy-to-use, yet e↵ective measurement tool. Hence,
we used two single-item scales, of which on is shown in
Figure 2.

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We used a hybrid-measures design. Each participant
read six texts with the same line spacing but six di↵erent
font sizes. Hence, for font size, we collect repeated measures, while for line spacing, we obtain between-group
data. The order in which the font sizes were presented
was counter-balanced to cancel out sequence e↵ects.

Literal questions could be directly answered from thetext such as “the giant turtle lives? (a) in the Seychelles, (b) Caribbean, etc.” Inferential questions required a deeper comprehension of the text (see Figure 3
in the paper). Using literal and inferential questions
resemble questions about the main idea and main facts
used by Dyson and Hazelgrove [16].

0.40– For line spacing we tested 0.8, 1.0, 1.4, and 1.8, where
1.0 represents the browser’s default line spacing –Firefox, in this case– which equals 120% points of the
font size.7 We chose 1.4 since many style guidelines
suggest to use slightly increased line spacings and 1.0
because is the default in word processors.

For each text, we created two questions: one literal andone inferential. Questions were asked right after eachtext had been read to avoid order and memory e↵ects,
e.g. the last text scoring a higher comprehension score.

0.30– For font size, we used the levels 10, 12, 14, 18, 22, and26 points. We chose 10, 12, and 14 points to compare
the results with previous studies [1, 2, 3, 7, 8]. The
larger font sizes were chosen to cover a wide range ofsizes, as previous work had indicated that larger fontsizes improves readability, without having shown thelimits of this improvement.

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In our experimental design, font size and line spacing
served as independent variables with 6 and 4 levels, respectively:Comprehension Score: To measure text comprehension, we used literal and inferential questions. That is,
we cover both types of text comprehension. Inferential
items are questions that require a deep understandingof the text content, because the question cannot be answered straight from the text. Literal questions, in contrast, can be answered directly from the text. We used
multiple-choice questions with four possible choices, one
correct choice, two wrong choices, and “I don’t know”.
To compute the text comprehension score, the correct
choice counted 100% and the rest 0%.

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Designincreases, the reader has encountered more difficulties,
which means that the text is more difficult to read.

Fixation
Fixation Duration
Duration Mean
Mean (ms)(ms)

sizes and line spacings. We chose Wikipedia, since it is
the most-frequently visited text-heavy website.6 Other
more frequently visited websites, such as Google, Facebook, or Yahoo, contain almost no text or are multimedia focused, hence not useful for this study. Readability and comprehension were analyzed via eye-tracking,comprehension tests, and subjective perceptions of theparticipants.

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or had already finished university degrees. All participants were frequent readers: per day, 47.83% read lessthan four hours per day, 39,13% read between four andeight hours per day, and 13.04% participants read morethan eight hours.

Segun lo que acabas de leer en la Wikipedia ‘According to what
you just read on Wikipedia:’

Materials– El gorila es un primate carnı́voro. ‘The gorilla is a carnivorous primate’.

To isolate the e↵ects of the text presentation, the texts
themselves need to be comparable in complexity. In thissection, we describe how we designed the texts that wereused as study material.

– El gorila tiene un ADN muy similar al de los humanos. ‘The
gorilla’s DNA is similar to the humans’ one.
– El gorila vive en los bosques del sur de África. ‘The gorilla
lives in the forests of southern Africa’.

– No lo sé, creo que lo no ponı́a o al menos yo no lo recuerdo.
‘I do not know, I think it was not in the text, or at least I
do not remember it’.
Figure 3. Comprehension item.

Wikipedia Entries

Since Wikipedia entries are heterogeneous, it is challenging to find many similar entries. We decided against
modifying articles to increase ecological validity. Thus,
we went through the articles of Wikipedia and chose 24
articles which share the following comparable characteristics:(a) All texts used in the experiment cover the same genre
and the same topic, namely animals. We chose animals because they are a topic of general interest, nottechnical or academic.

1024x768 pixels. The time measurements of the eyetracker have a precision of 0.02 seconds. The eye-tracker
was calibrated for each participant and the light focus
was always in the same position. The distance between
the participant and the eye-tracker was constant (approximately 60 cm. or 24 in.) and controlled by using
a fixed chair. Figures 4 and 5 show the setup as usedduring the study.

(b) They all have a similar number of words in the first andthe second paragraphs, ranging from 40 to 60 words
for each of the paragraphs.
(c) They have a similar discourse structure: title, the firstparagraph presents the animal and the place where it
lives, and the second and third paragraphs provide
more details.
(d) The layout was always the same: the paragraphs werelocated in roughly the same position of the screen.
Each article contained one image on the top-right ofthe content pane (see Figure 1).
(e) All texts had low frequencies (ranging from two to five)of numerical expressions, acronyms, and foreign words,
because these type of words are processed di↵erently
than regular words [15, 31].
(f) All the entries used the sans-serif font Arial, which
Wikipedia uses as default on Firefox and otherbrowsers on MS Windows.
For each of the selected Wikipedia articles, we obtained
the HTML source code. To alter the presentation, weused a browser plug-in (StyleBot) to modify the style
sheet (CSS) to change font size and line spacing.
Comprehension Questionnaires

The comprehension questions were administered in formof questionnaires. There was one questionnaire for eachof the Wikipedia articles containing six multiple-choicequestions. An example of each type of item is given in
Figure 3.
Equipment

The eye-tracker that we used was the Tobii 1750 [33],which has a 17-inch TFT monitor with a resolution of

Figure 4. Setup: the eye-tracker used during the experiment showing a Wikipedia article used.

Procedure

The study took place at the end of 2012. The sessions
were conducted at the Universitat Pompeu Fabra, andlasted around 20 minutes for each participant. Each session took part in a quiet room, where only the interviewer (first author) was present, which ensured that theparticipants could concentrate. Each participant performed the following four steps.
First, we handed out the questionnaire that was designed
to collect demographic information: age, gender, native
languages, education, and reading hours per day. Second, we asked the participants to read six Wikipedia
articles in silence. For each article, the participants readfreely covering at least the first 3 paragraphs. During
this time, the reading was recorded by the eye-tracker.
After finishing each article, the participants completed
the corresponding comprehension questionnaire. Finally,

Font size
10
12
14
18
22
26

Mean
.255
.241
.224
.208
.199
.204

SD
.059
.054
.048
.040
.037
.045

Table 1. Mean and standard deviation of the average fixation durations per font size. Smaller mean values indicate
better readability.

(⌘ 2 = 0.159) suggests a high practical significance.
Holm-corrected pairwise, repeated-measures t-tests revealed the following significant di↵erences:– For 10 points font size, participants had significantly
longer fixation durations than for all larger font sizes(p < .01 for 12 pts, p < .001 all other comparisons).

Figure 5. Heat map of the eye fixations.

we presented all the articles again to the participants,
and asked them to provide their ratings regarding the
texts readability and comprehension.
RESULTS– For 12 points font size, participants had significantly
longer fixation durations that for all larger font sizes(p < .001, each).
– For 14 points font size, participants had significantly
longer fixation durations than for all larger font sizes(p < .001, each).
– For 18 points font size, participants had significantly
longer fixation durations than for 22 points (p = .003).

In this section, we present the analysis of the data fromthe eye-tracker (fixations), the comprehension tests, andOtherwise, there were no significant di↵erences bethe subjective perception
ratings.
muy difícilmuy– fácil
tween the larger font sizes.
‘very
easy’
‘very difficult’Mean Fixation Duration

2
3
4
5The data indicates an absolutely continuous decrease of
Facilidad
comprensión
Font Size:
Inmuy
somedifícil
cases,1 the eye-tracking
did not funcmuy fácil
the mean fixation duration until font size 18. Mean fixcorrectly throughout the whole study. Hence, we‘Easetionof comprehension’ation duration was lowest for 22 points.
‘very
easy’
‘very
difficult’
only analyze the 93 participants that successfully con-

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Figure 6. Mean
fixation
duration(lower fix10
12
14
18 by font
22 size 26
ation durations indicate better readability).
Font
Font Size
Size

A Two-Way ANOVA revealed a significant main e↵ectof font size on mean fixation duration (F (5, 445) =66.825, p < .001). The Eta-Square e↵ect size value

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Fixation
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0.20Fixation
0.30Duration
0.40Meanof the font size conditions.

Spacing: Figure 7 and Table 2 show the mean fixation
duration for each of the line spacing conditions. The
number of participants per condition were 24, 29, 26, and25 for the conditions 0.8, 1.0, 1.4, and 1.8 respectively.
We did not find a significant ●●e↵ect of line
spacing on●●
fixation duration (F (3, 89) = 0.064, p = .978).
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0.8

0.10

1
2
3
4
tributed
to all six font-size
conditions.
Figure6 and
Ta- 5Facilidad
comprensión
1 show the mean durations of eye fixations for each
‘Ease ofble
comprehension’

1
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0.8

1

1.4

1.8

Line Spacing
Spacing
Line

1.4

1.8

Line Spacing
SpacingLine

Figure 7. Mean fixation duration by line spacing (lower
fixation durations indicate better readability).

SD
.046
.057
.054
.050

Comprehension Score

Table 2. Mean and standard deviation of the average
fixation durations per line spacing. Smaller mean values
indicate better readability.

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12

14

18

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– For 12 points, participants had significantly lower
comprehension scores than for 26 points (p < .05).
Hence, our data indicates that, comprehension was
negatively a↵ected by small font sizes.
100%!
80%!
60%!
Wrong!

40%!

Correct!

20%!
10!

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10

– For 10 and 12 points, participants had significantly
lower comprehension scores than for 18 points (p <.01, both).

0%!

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1.4
1.8
0.8
1

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Fixation Duration (mean)

Interaction Fontsize x Spacing: There was asignificant interaction between font size and spacing(F (15, 445) = 4.098, p < .001). The Eta-Square e↵ect
size value (⌘ 2 = .034) suggests low practical significance.
In the interaction plot (Fig. 8) with Spacing as group,
we see all 4 lines dropping in parallel for increasing fontsize. Only for 26 points, the lines visibly diverge. We ran
tests on subsets of the data. The interaction e↵ect disappears when only keeping font sizes 10-18. A marginale↵ect remains when keeping font sizes 10-22. This indicates that the two largest tested fonts (22 and 26 points)
were a↵ected by line spacing.

Figure 9 shows the comprehension score distribution
for each of the font size conditions. A Levene Test
showed that the variances in the comprehension scores
were not sufficiently equal to use parametric statistics(F (5, 532) = 5.696, p < .001). A Friedmann Test revealed a significant e↵ect of font size on the comprehension score ( 2 (5) = 27.29, p < .001). Holm-corrected
pairwise Wilcoxon Signed-Rank tests revealed the following di↵erences:

Correct Responses!

Mean
.224
.220
.220
.223

26

Fontsize
Figure 8. Mean fixation duration interaction plot.

Number of fixations: While mean fixation duration isan established proxy for readability, it might be assumed
that shorter fixations led to more fixations, which intuitively would appear as counter-evidence to any readability improvement. Hence, we analyzed the number offixations. A Two-Way ANOVA revealed the existenceof a significant main e↵ect of font size on the numberof fixations (F (5, 445) = 5.249, p < .001). However, theEta-Square e↵ect size value (⌘ 2 = .025) suggests low
practical significance. The post-hoc comparisons, using
holm-corrected, pairwise t-tests, revealed only a single
significant di↵erence: the number of fixations were significantly lower for 10 points (M = 115.7, SD = 61.4)
than for 12 points (M = 149.4, SD = 87.3) (p < .001).
Other comparisons are not significant. Neither plots nor
post-hoc tests do not reveal a clear trend. Therefore, thereduced mean fixation duration observed for increasing
font size cannot be explained by a an increase in thenumber of fixations.

12!

14!
18!
Font Size!

22!

26!

Figure 9. Mean comprehension scores by font size.

Figure 10 shows the comprehension score distribution
for each of the line spacing conditions. A Levene Test
showed that the variances in the comprehension scores
were not sufficiently equal to use parametric statistics(F (3, 534) = 6.729, p < .001). A Kruskal-Wallis Test revealed a significant e↵ect of line spacing on the comprehension score ( 2 (3) = 19.56, p < .001). Holm-corrected
pairwise Mann-Whitney U tests revealed the followinge↵ects:– For 0.8 line spacing, participants had significantly
lower comprehension scores than for 1.0, 1.4. and 1.8(p < .05, p < .05, p < .001, respectively).
100%!
Correct Responses!

Spacing
0.8
1.0
1.4
1.8

80%!
60%!
Wrong!

40%!

Correct!

20%!
0%!
0.8!

1!
1.4!
Line Spacing!

1.8!

Figure 10. Mean comprehension scores by line spacing.

Hence, in our data set, the smallest line spacing led
to lower scores in the text comprehension tests.

Figure 11 shows the distribution of the subjective readability ratings by font size. A Levene Test showed thatthe variances in the perception ratings were not sufficiently equal to use parametric statistics (F (5, 363) =6.705, p < .001). A Friedman Test revealed a significant e↵ect of font size on subjective readability ( 2 (5) =135.85, p < .001). Holm-corrected pairwise Wilcoxon
Signed-Rank tests revealed the following di↵erences:– For 10 points, readability ratings were significantly
lower than for all other sizes (p < .001, each).
– For 12 points, readability ratings were significantly
lower than for all larger sizes (p < .001, each).
– For 14 points, readability ratings were significantly
lower than for 18 points (p < .01).
– For 26 points, readability ratings were significantly
lower than for 18 points (p < .05).
Readability Rating!

100%!
80%!
very good!

60%!

good!

40%!

neutral!
bad!

20%!

very bad!

0%!
10!

12!

14!
18!
Font Size!

22!

Thus, for the participants of this study, the subjective
perception of readability continuously improved
and was highest for 18 points font size.
Figure 12 shows the distribution of the readability ratings by line spacing. A Levene Test showed that the variances in the perception ratings were sufficiently equal touse parametric statistics (F (3, 365) = 1.568, p = .197).
A independent-measures ANOVA did not reveal any significant e↵ect on the subjective readability (F (3, 365) =2.074, p = .103).
Readability Rating!

100%!
80%!
very good!

60%!

good!

40%!

neutral!
bad!

20%!

very bad!

0%!
1!
1.4!
Line Spacing!

– For 10 points, comprehension ratings were significantly lower than for all larger fonts (p < .001, each).
– For 12 points, comprehension ratings were significantly lower than for all larger fonts (p < .001, each),
as well.
– No significant di↵erences were found between 10 and
12 points.
– Similarly, no significant di↵erences were found between 14, 18, 22, and 26 points.
100%!
80%!
very good!

60%!

good!

40%!

neutral!
bad!

20%!

very bad!

0%!
10!

26!

Figure 11. Subjective readability ratings increased withincreasing font sizes until 18 points.

0.8!

the variances in the perception ratings were sufficiently
equal to use parametric statistics (F (5, 363) = 2.189, p =.055). There was a significant e↵ect of font size on comprehension ratings (F (5, 363) = 18.614, p < .001). The
eta-square e↵ect size (⌘ 2 = .193) indicates large practical significance. Holm-corrected pairwise, repeatedmeasures t-tests revealed the following e↵ects:

Comprehension Rating!

Subjective Readability Ratings

1.8!

Figure 12. No significant e↵ect of line spacing on readability was found.

Comprehension Perception Ratings

Figure 13 shows the distribution of subjective comprehension ratings by font size. A Levene Test showed that

12!

14!
18!
Font Size!

22!

26!

Figure 13. Subjective comprehension ratings increase
with increasing font sizes.

Hence, in our study, subjective comprehension of
the texts was significantly lower for small fontsizes (10 and 12 points) compared to the larger tested
font sizes.
Figure 14 shows the distribution of comprehension ratings by line spacing. A Levene Test showed that the variances in the perception ratings are sufficiently equal towere parametric statistics (F (3, 365) = 2.209, p = .087).
There was a significant e↵ect of line spacing on comprehension ratings (F (3, 365) = 3.249, p = .022). The
eta-square e↵ect size (⌘ 2 = .018) indicates low practical significance. Holm-corrected pairwise t-tests revealed
the following e↵ects:– For 1.0 comprehension ratings were significantly
higher than for 1.8 (p < .05).
These results hint that our participants felt that theircomprehension was impaired by the largest line
spacing.
DISCUSSION
Font Size

Font size had significant e↵ects on all dependent measures. The observed e↵ects are consistent.
The average fixation durations decreased steadily until
22 points, which indicates that readability improved withincreasing font size. The subjective measures confirmed

Comprehension Rating!

100%!
80%!
very good!

60%!

good!

40%!

neutral!
bad!

20%!

very bad!

0%!
0.8!

1!
1.4!
Line Spacing!

1.8!

Figure 14. Subjective comprehension ratings were higher
for 1.0 than 1.8 line spacing.

the objective measures: subjective readability was higher
for the larger font sizes (14 to 26 points) than for thevery small sizes (10 and 12 points), and it was best for
18 points. The results are inline with previous work
comparing font sizes [1, 3, 4, 5, 6, 8], where the largest
font size was found to be the best, either in terms ofreading time or preference. However, those works onstudied font sizes up to 14 points. Thus, it was not clear
whether the readability of texts would improve beyond
14 points, and at what point the letters would become
too large so that the e↵ects get reversed.
Beyond readability, we also showed that comprehension
was impaired by smaller font sizes. Our subjects gave
more wrong answers for 10 and 12 than for 18 and 26
points. This shows that the measured, objective readability translated into measurable, objective improvement of comprehension. This is a notable insight, as 10
and 12 points happen to be font sizes which historicallywere very commonly used in websites [20].
Thanks to testing font sizes beyond the one that weretypically studied in the past, we showed that the improvement in readability continues with increasing fontsize beyond 14 points. However, since no further significant improvements were observed beyond 22 points,
and 18 points scores the best subjective readability. The
results indicate that a local maximum might for objective readability exist between 18 to 26 points.8 A local
maximum is to be expected, as increasing font size will
required to have less and less text in a single line, whichleads to more frequent eye jumps, scrolling, and the lossof overview [17].
This results could also be expected for target populations
with reading disorders such as people with dyslexia. In
fact, we replicated this experimental setting with smallergroup of 28 people who were diagnosed dyslexia andfound similar e↵ects of font size [29].
These findings advance our knowledge in two important
aspects: first, beyond readability, this study is –to thebest of our knowledge– the first to prove an e↵ect of fontsizes on both, objective and subjective readability andcomprehension for a general target population. Second,8

For the fixation duration the minimal value was attained for
22 points.

since we tested larger font sizes as used in previous studies, we could show that the positive e↵ect continues until
18 points, before it flattens out. This finding is in contrast with common recommendations, which suggest touse 10, 12, or 14 points.
Line Spacing

The e↵ects of line spacing were less pronounced. Our
study revealed significant e↵ects on comprehension, but
not on readability.
Objective comprehension was lower for small line spacings: in the 0.8 line spacing condition, less comprehension questions were answered correctly. Further, subjective comprehension was higher for the standard spacing
compared to the largest spacing (1.8). Thus, our study
provides evidence that comprehension of texts may be
impaired when line spacings are too small or too large.
Yet, the data did not reveal e↵ects as pronounced as
by font size. This corroborates the assumption by Bix[9] that line spacing is not a major factor on readability
and that the ideal line spacing depends on other factors.
Nevertheless, our work extends previous work in one important aspect: while previous work argues over readability, this work is –again to the best of our knowledge–
the first to show that line spacing a↵ects the comprehension of texts as well.
Limitations of the Study

One of the limitations of our study is that we only considered the first three paragraphs of Wikipedia articles.
When using eye-tracking to study reading, it has beenfound that the initially measured fixation durations are
longer, since users are still in a familiarization phase [22].
The heat map in Figure 15 shows that this e↵ect occurred in our setup, too. However, the heat map also
shows that the fixation durations normalize when reading on. Yet, since we assume that people often only readparts of web pages, we conclude that despite the shortlengths of the texts, our findings have high ecologicalvalidity, that is, this familiarization also happens whenpeople read web pages [11].
In comparison to other previous work [1, 3, 4, 5, 14],
we did not measure reading time. We did so for two
reasons. First, we wanted to create a natural setting,in which reading as fast as possible is neither a goal
nor an indicator for readability. Second, reading fastcan, in our opinion, be misleading. For example, in caseof bad readability, participants might become frustrated
and start skimming the text instead of reading it withfull attention. Our decision is backed up by findings
from Beymer et al. [5], who found significantly longer
fixations for smaller fonts but no significant e↵ect offont size on reading speed. Since we used comprehension tests, our evidence indicates that participants did
not skim texts.
Another limitation of our study is that we used a
fixed line length, as the browser window was maximized
throughout the study. Previous research [16, 32] hasmight be equally well to ensure readability and comprehension.
CONCLUSIONSWe tested the e↵ect of font size and line spacing on objective and subjective readability and comprehension of
Wikipedia articles. Up to a font size of 18 points, subjective and objective readability as well as comprehension
improved continuously. Beyond 22 points, there were nofurther e↵ects for the objective measures, and a decreasein subjective readability. Line spacing, in contrast, hadno e↵ect on the objective readability, but extreme spacings (0.8 and 1.8) negatively a↵ected objective and subjective comprehension.

Figure 15. Heat map of a Wikipedia article used in the
study (18 points and 1.0 lines for line spacing).

found that line length a↵ects reading, and in the realworld, people may shrink or enlarge the browser window
freely. Yet, the typical browser will not change its window size when changing the font size. Some websites use
a fixed width to display texts. Hence, our design has
high ecological validity and allows applying our findings
to typical reading settings.
Previous work [3, 10] has shown that readability of texts
also depend on the font type. Since we only used a single
font, namely Arial, our findings might not be generalize
to other font types. However, Arial is one of the mostwidely encountered fonts in the web, as it is the defaultsans-serif font in most modern web browsers. More important, we believe that our work shows a clear indication that bigger sizes of similar font types lead to better
reading and comprehension, encouraging designers to,regardless of the font type, think about and argue forbigger font sizes.
RECOMMENDATIONS

On the basis of our results, we recommend to use atleast 18-point font size for the text body of websites.
This value strikes the balance between having the best
readability, comprehension, subjective perception scores,
and allowing to fit as much text on the screen as possible. In our experiment, increasing font size beyond 22
points led to no significant improvements. Best subjective readability was achieved with 18 points. This forms
a stark contrast to existing recommendations and guidelines, which typically suggestion font sizes from 10 to 14
points.
Regarding line spacing, our data suggests that it is best
not to deviate too much from the standard line spacing(1.0). However, moderately larger line spacing, such as
the widely used 1.5 spacing (e.g. in journal manuscripts),

Our work advances previous knowledge, as it is the firstwork (1) to study reading with a general population in
the context of the Web and (2) to show that readability improves past the typically studied font sizes (10, 12,
and 14 points). It demonstrates that a simple increase in
font size is a cheap and efficient way to improve access to
textual information. Thus, it is a great and welcome development, that today’s (2016) browsers, such as Firefox
or Chrome, ship with a default font size of 16 points.
Future work needs to explore whether these findings
are stable when other parameters, such as font type orcolumn width, are altered. In particular, since more
and more reading is taking place on tablets and mobile
phones with much smaller screens, additional studies are
required to verify our findings for those devices.
ACKNOWLEDGMENT

We thank the participants of the study.
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