THE METRIC SYSTEM 

T HE METRIC CONVERSION ACT OF 1975- 

P.L. 94-168, amended by P.L. 100-418 (August 

23, 1988)-stated that the policy of the United States 

is to designate the metric system as the preferred 

system of weights and measures for United States 

trade and commerce. Reference is to the International 

System of Unit@) or SI (from the French "Le Systeme International d'Unit6s") as modified by the 

Secretary of Commerce for use in the United States. 

SI differs from earlier versions of the metric system 

in that (1) the base units are more accurately defined 

and (2) specific directives and guidelines are provided 

for use of prefixes and for the development of combined or derived units. SI and the history of its development are described in National Bureau of Standards 

Special Publication 330 (as revised). Other helpful and 

authoritative references for the use of SI are Publication E380 (as revised) of the American Society for 

Testing and Materials; Publication 268 (as revised) of 

the American National Standards InstituteIInstitute 

of Electrical and Electronic Engineers; Publication 

85-1, Metric Editorial Guide (fourth edition revised), 

April 1985, American National Metric Council; and 

Federal Standard 376A, Preferred Metric Units for 

General Use by the Federal Government, General 

Services Administration. 

Aside from the fact that SI units simplify measurements and calculations, a major scientific advantage 

of SI is that it eases the exchange of data in the 

many disciplines that have used inchlpound (U.S. 

customary) units of measure. The term "inchlpound 

units" not only includes units based on the inch and 

the pound commonly used in the United States but 

also includes all other (nonmetric) units not considered 

part of SI. 

In adopting the Metric Conservation Act, the 

United States officially expressed its intent to join 

other nations in the use of SI. The amended act expressly stated that the transition to the use of the 

metric system by the Federal Government shall be 

implemented by the end of fiscal year 1992, except to 

the extent that such use is impractical or is likely to 

cause significant inefficiencies. 

To ensure timely and effective compliance with 

P.L. 94-168, the U.S. Geological Survey would be 

ready by FY 1991 to begin extending the requirement to cover all new scientific reports published in 

Table 5. Conversion factors for SI (metric) and 

inchlpound (U.S. customary) units of measurement 

[SI (International System of Units) a modernized metric system of 

measurement. An asterisk after the last digit of the factor indicates 

that the conversion factor is exact and that all subse uent di 'ts are zero; all other conversion factors have been rounted to gur significant digits. Use of hectare (ha) as an alternative name for 

square hectometer (hmz) is restricted to the measurement of small 

land or water areas. Use of liter (L) as a special name for cubic 

decimeter (dms) is restricted to the measurement of liquids and 

gases. No prefix other than milli should be used *th liter. Metric 

ton (t) as a name for mepgram (Mg) should be restr~cted to cornmer- cial usage, and no prefixes should be used with it] 

the official book series, provided the requirement does 

not conflict with cooperators' requirements nor 

detract from the clarity of reports directed to mixed 

audiences (interdisciplinary scientists, legislators, 

technical personnel such as engineers, and nonscientific personnel such as planners and the general 

public). 

All other publications series contain subject matter 

of differing technical complexity directed to readers 

of varying technical sophistication. Products run the 

gamut from lay-reader, information-type releases to 

complex mathematical treatises, and the selection of 

either SI, inchlpound, or dual units for a publication 

in one of these series is the author's responsibilitywith guidance from appropriate Division staff. The 

decision to use a system of units should be made in 

the planning stage of a publication and not when 

project activities are near completion. This decision 

is especially important where SI or dual units are to 

be used, because it enables project personnel to 

familiarize themselves with what may be a new suite 

of units, and it improves the accuracy of published 

data. If dual units are used, the numbers used first 

should be the ones that the measurements were made 

in. The Survey, however, discourages dual usage. 

In light of the transition to SI, the use of conversion tables is encouraged in Survey publications to 

expose readers to SI and to help familiarize readers 

with the SI units that correspond to the inchlpound 

units commonly used by the Survey. 

The most often read SI and inchlpound units, and 

factors for their conversion, are given in table 5 

(less common conversions are found in most good 

dictionaries). 

122 The metric system 
Table 5. Conversion factors for SI (metric) and 

inchlpound (U.S. customary) units of 

measurement-Continued 

Table 5. Conversion factors for SI (metric) and 

inchlpound (U.S. customary) units of 

measurement-Continued 

A. Factors for converting SI metric units to incUpound units - -- 

To convert from To Multiply by 

Length 

B. Factors for converting incUpound units to SI metric units 

To convert from To Multiply by - 

Length 

millimeter (mm) inch (in) 0.03937 

meter (m) foot (ft) 3.281 &;) 1.094 kilometer (km) 0.6214 

mde, nautical (nmi) 0.5400 

Area 

meter2 (m') foota (fix) 10.76 

yardP (yd') 1.196 acre 0.0002471 

hectometer' (km') acre 2.471 

kilometer' (kmP) mileP (mi') 0.3861 -. 

Volume - 

centimete? (cms) 

decimeters (dms) 

inchs (ins) 

inchS (ins) 

pint (pt) 

quart (qt) fZ2f 

pl;nY) 

arrel ( bl), (petroleum, 

1 bbl=42 gal) acre-foot (acre-ft) 0.0008107 

hectometers (ha) acre-foot (acre-ft) 810.7 

kilometers (kms) mile"mia) 0.2399 

Volume per unit time (includes flow) 

decimeters per second foots per second (ftsls) 0.03531 

(dmsls) gallon per minute 15.85 

- (gall&) 

barrel Der day (bblld). 

(petrbleumj 

meter3 per second footS per second (ftals) 35.31 

(msls) gallon per minute 15.850 

kidlmin) 

Mass 

vem '.Ki ounce uvuirdu poi? II.~?. #::I,IJ;~?~ 

inch (in) millimeter (mm) 25.4* 

foot (ft) meter (m) 0.3048 

st &;I 

meter (m) 0.9144* 

kilometer (km) 1.609 

mile, nautical (nmi) kilometer (km) 1.852* 

Area 

foota (ft') meter' (ma) 0.09290 

yard' (yd') meter' (mP) 0.8361 acre meter2 (ma) 4,047 

hectometer2 (hd) 0.4047 

mile2 (mi') -- - kilometerP (ha) 2.590 

Volume 

incha (id) centimeters (cma) 16.39 

decimeter"dma) 0.01639 

foots (fts) decimetera (dma) 28.32 

mete? (ma) 0.02832 

yarda (yda) mete? (ma) 0.7646 

pint (pt) decimeters (dms) 0.4732 

quart (qt) decimeters (dma) 0.9464 

gallon (gal) decimetera (dms) 3.785 

meterS (ma) 0.003785 

barrel (bbl), ( tro meterS (ma) 0.1590 

leu. 1 bbc42 &) 

acre-foot (acre-ft) meters (mS) 1,233 hectometers (ha) 0.001233 

mile"(mis) kilometerS (ha) 4.168 

Volume per unit time (icludes flow) 

footS per second (ftals) decimeterS per second 28.32 

(dmSls) 

meters per second 0.02832 

(msls) 

gallon per minute decimeterS per second 0.06309 

(gallmin) (dm'ls) metera per second 0.00006309 

(msls) 

barrel per day (bblld), decimeters per second 0.001840 

(petroleum) (dmsls) 

hi865 

The metric system