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SOP No. 12 

Recommended Standard Operating Procedure 

for 

Calibration of Metal Tapes1

Tape-to-Tape Method 

I. Introduction 

1.1. Purpose 

Metal tapes are used by contractors, surveyors, and others for building layouts, 

measurement of land areas, establishment of land boundaries, and similar 

purposes. The accuracy of the measurement often must be provable to 0.000 1 

meter on a 100 meter tape (0.000 1 foot on a 100 foot tape), sometimes in a court 

of law. A significant feature of such proof is the knowledge of the accurate length 

of the tape used. Inaccuracies in such measurements can cause structural 

misalignments, boundary controversies, and other problems. The test method 

described here provides a procedure to calibrate such tapes by comparison to a 

calibrated standard tape. 

1.2. Prerequisites 

1.2.1. Valid calibration certificates with appropriate values and uncertainties 

must be available for all of the standards used in the calibration. All 

standards must have demonstrated metrological traceability to the 

international system of units (SI), which may be to the SI through a 

National Metrology Institute such as NIST. 

1.2.2. The operator must be trained and experienced in precision measuring 

techniques with specific training in GMP 8, GMP 9, SOP 12, and SOP 29. 

1.2.3. Laboratory facilities must comply with following minimum conditions to 

meet the expected uncertainty possible with this procedure. Equilibration 

of tapes requires environmental stability of the laboratory within the stated 

limits for a minimum of 24 hours before a calibration. 

 

1

 Many references are provided in in-pound units due to common use in U.S. weights and measures requirements. 

The International System of Units, SI, is the official system of units for metrological traceability. 
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Table 1. Environmental conditions. 

Temperature Requirements During a Calibration Relative Humidity 

(%) 

Lower and upper limits: 18 °C to 22 °C 

Maximum changes: < ± 1 °C / 24 h and ± 0.5 °C / 1 h 40 to 60 ± 10 / 4 h 

2. Methodology 

 2.1. Scope, Precision, Accuracy 

The method is applicable to the calibration of metal tapes such as used by 

surveyors, builders, and contractors. The overall length and specified intermediate 

lengths may be checked by the technique. The accuracy is limited by the accuracy 

of the calibration of the standard tape and by the precision of intercomparison. 

The latter should be within ± 0.001 foot, corresponding to 10 parts per million in 

a 100 foot tape. The method is limited to calibration of steel tapes (because the 

tension is specified as the result of a 10 pound load). 

2.2. Summary 

This procedure is based upon the method developed by C. Leon Carroll Jr., 

National Bureau of Standards, NBSIR 74-451, “Field Comparisons of Steel 

Surveyors’ Tapes.” 

The tape to be calibrated is stretched out parallel to a standard tape on a 

reasonably flat surface. Paper scales (graph paper), graduated in millimeters are 

used at the zero and at each specified interval of calibration to measure any 

difference between the two tapes. The length of the tape undergoing calibration is 

computed from the known length of the standard tape and the observed 

differences between the test tape and the standard. 

Calibrations are usually made at each 1 foot interval for the first 10 feet, and at 

each 10 foot interval to the full length of the tape. 

2.3. Apparatus/Equipment Required 

2.3.1. Standard tape, calibrated to within ± 0.001 foot. 

2.3.2. Pieces of graph paper (10 x 10 to the centimeter, i.e., millimeter 

graduations), approximately 5 cm in width by 15 cm in height. Number 

the horizontal centimeter graduations, 0, 1, 2, etc.. Evaluate and document 

the range of use to within 0.1 mm using a calibrated rigid rule or a 

microscope with calibrated reticles to ensure that the graph paper has 

sufficient accuracy. 
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2.3.3. Equipment, see Figure 1, to apply tension to the tapes under test, 

consisting of the following. 

2.3.3.1. Spring scales (two); one capable of indicating a load of 

10 pounds and the other capable of indicating a load of 

20 pounds. The scales should be calibrated with an 

accuracy of ± 0.1 lb. This may be done by the arrangement 

shown in Figure 2 with results internally documented. 

2.3.3.2. Turnbuckles, suitable for adjusting tension on the tapes. 

2.3.3.3. Swivel connectors to prevent axial twisting of the tapes. 

2.3.3.4. Magnifying glass to aid in reading the graph paper values. 

2.3.3.5. Small weights (< 5 lb) to hold the tapes down flat to the 

paper. 

Figure 1. Experimental Arrangement. 

 

 
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Figure 2. Calibration of Spring Scales. 

2.4. Symbols 

Table 2. Symbols used in this procedure. 

Symbol Description 

S Length of the Standard Tape at the calibration interval 

LX Length of the Test Tape at the calibration interval 

cf Conversion factor, 

0.032 808 ft/cm for tapes graduated in feet 

0.010 000 m/cm for tapes graduated in meters 

Ln Nominal length of tape interval under test 

AE Cross-sectional area times Young’s modulus of elasticity 

Q0 Lower load (tension) applied to the tape 

Q1 Higher load (tension) applied to the tape 

L0 Length of tape under load Q0

L1 Length of tape under load Q1

2.5. Procedure 

2.5.1. Inspect the tapes to ensure that no kinks, dents, or other damage are 

present which will affect the accuracy of the calibration. 

2.5.2. Clean the tapes by first wiping with a soft cloth, and then with a soft cloth 

saturated with alcohol to remove protective oil film. 
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2.5.3. Stretch the test tape and standard tape parallel to each other on a 

reasonably flat surface. The evenness of the surface is less important than 

the parallelism of the tapes. The two tapes should be separated by a 

constant distance of about 1 to 3 centimeters. The zero and test intervals of 

the two tapes should not be in coincidence but rather displaced by one or 

two centimeters, as indicated in the detail of Figure 1. 

2.5.4. Use the turnbuckles to apply equal loads of 10 pounds to the two tapes as 

indicated by the spring scales. (Note the use of swivels to prevent axial 

twisting.) 

2.5.5. Place a piece of graph paper under the zero interval and each interval to be 

calibrated as shown in the detail in Figure 1. Adjust the tapes and the 

paper so that the former are precisely aligned with the lateral rulings of the 

paper. It is convenient but not necessary for these to be the bold centimeter 

rulings of the paper. Note the amount of separation of the tapes at the zero 

interval and make corresponding adjustments at each calibration interval 

of interest. In this way, parallelism of the two tapes is easily verified. 

2.5.6. Make final adjustment of tensions on the tapes and recheck for parallelism 

at all test points before taking the readings described in 2.5.7. Do not 

disturb during the measurement sequence. 

2.5.7 Read the distances A, B, C, and D as indicated in the detail of Figure 1. 

Note that A and B are for the zero (or first) interval and are the same for 

all test intervals. C and D have subscripts 1, 2, etc. corresponding to the 

interval, i, calibrated. Make all readings to the center of the graduation 

mark tested and estimate to the closest 0.1 mm. Record all readings in 

centimeters. 

2.5.8. Record all measurements as Trial 1. 

2.5.9. Release the tension to the tapes and reapply it. 

2.5.10. Displace each piece of graph paper a few millimeters, then readjust the 

load, check for parallelism, and record a second series of measurements as 

Trial 2. 

2.5.11. Release the tension to the tapes and reapply it. 

2.5.12. Displace each piece of graph paper a few millimeters, then readjust the 

load, check for parallelism, and record a second series of measurements as 

Trial 3. 

2.5.13. After all measurements are completed; apply a thin film of oil to the tapes. 
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3. Calculations 

 3.1. Calculate and record A – B – C + D for each trial, then record the value of R, the 

range of these values (difference of highest and lowest) for each scale interval. 

The range should not exceed 0.15 cm. Sum the values for A, B, C, D for the three 

trials to use when calculating the length, L, of each interval. 

 

 3.2. The value obtained from ƩA – ƩB – ƩC + ƩD must equal the sum of the column 

A – B – C + D, otherwise an error has been made in the calculations. 

 3.3. Calculate the length of the test tape at each calibration interval according to the 

following equation. 

 ( ) 3 A B C D cf LX  S      (1) 

 

4. Measurement Assurance 

4.1. Duplicate the process with a suitable check standard or have a suitable range of 

check standards for the laboratory. See NISTIR 7383 SOP 17, SOP 20 and 

NISTIR 6969 SOP 30. Plot the check standard length and verify it is within 

established limits OR a t-test may be incorporated to check the observed value 

against an accepted value. The mean of the check standard observations is used to 

evaluate bias and drift over time. Check standard observations are used to 

calculate the standard deviation of the measurement process which contributes to 

the Type A uncertainty components. 

4.2 If a standard deviation chart is used for measurement assurance, the standard 

deviation of each combination of 3 Trials is calculated and the pooled (or 

average) standard deviation is used as the estimate of variability in the 

measurement process. Note: the pooled or average standard deviation over time 

will reflect varying conditions of test items that are submitted to the laboratory. A 

standard deviation chart will be needed for each interval calibrated (at least 

initially) so that the variability resulting from transfers will be measured. 

5. Assignment of Uncertainty 

5.1. The limits of expanded uncertainty, U, include estimates of the standard 

uncertainty of the length standards used, us, estimates of the standard deviation of 

the measurement process, sp, and estimates of the effect of other components 

associated with this procedure, uo. These estimates should be combined using the 

root-sum-squared method (RSS), and the expanded uncertainty, U, reported with 

a coverage factor to be determined based on degrees of freedom, which if large 

enough will be 2, (k = 2), to give an approximate 95 percent level of confidence. 

See NISTIR 6969, SOP 29 (Standard Operating Procedure for the Assignment of 
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Uncertainty) for the complete standard operating procedure for calculating the 

uncertainty. 

5.1.1 The expanded uncertainty for the standard, U, is obtained from the 

calibration report. The combined standard uncertainty, uc, is used and not 

the expanded uncertainty, U, therefore the reported uncertainty for the 

standard will usually need to be divided by the coverage factor k. 

5.1.2. The standard deviation of the measurement process, sp, is taken from a 

control chart for a check standard or standard deviation charts. (See SOP 

17, SOP 20, and SOP 30) 

5.1.3. Uncertainty associated with bias, ud. Any noted bias that has been 

determined through analysis of control charts and round robin data must 

be less than limits provided in SOP 29 and included if corrective action is 

not taken. See SOP 29 for additional details 

5.1.4. Other standard uncertainties usually included at this calibration level 

include uncertainties associated with the ability to read the graph paper, 

only part of which is included in the process variability due to parallax and 

visual capabilities, and uncertainties associated with the graduations of the 

graph paper. 

 Table 3. Example uncertainty budget table. 

Component Description Reference 

us Standard uncertainty for standards Calibration report, divide by k 

sp Standard uncertainty for the process Measurement assurance process; 

range charts 

ugp Standard uncertainty for graph paper Must be assessed experimentally 

utw Standard uncertainty for the spring scales Must be assessed experimentally 

or from a calibration certificate 

ud Standard uncertainty for disparity due to 

drift/bias 

Rectangular distribution and 

reasons, 0.577 d, 0.29 d; 

SOP 29 (NISTIR 6969) 

ures Standard uncertainty due to resetting of 

the tapes 

Must be assessed experimentally 

uo Standard uncertainty for other factors 

6. Report 

Report results as described in SOP No. 1 Preparation of Calibration Certificates. 

 
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Appendix A 

Tape-to-Tape Method Data Sheet 

Date Environmental parameters 

Metrologist Before After Unc/ability to measure 

Test No. Temperature °C °C 

sp in Pressure mmHg mmHg 

df Humidity % % 

Based on NISTIR 6969, SOP 29, Appendix A at 95.45 % probability distribution: k factor 

 ID Range 

S 

X 

Tension lb 

Interval Trial A B C D A-B-C+D 

 1 

( ) 3A B C D cf 2     S L 

3 

Ʃ 

 Range 

 1 

( ) 3A B C D cf 2     S L 

3 

Ʃ 

 Range 

 1 

( ) 3A B C D cf 2     S L 

3 

Ʃ 

 Range 

 1 

( ) 3A B C D cf 2     S L 

3 

Ʃ 

 Range 

 1 

( ) 3A B C D cf 2     S L 

3 

Ʃ 

 Range 

 1 

( ) 3A B C D cf 2     S L 

3 

Ʃ 

 Range 

 
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Appendix B 

Supplemental Information 

B.1. Cleaning 

To clean a steel tape before calibration, first wipe the tape with a soft cloth; then with a 

soft cloth saturated with alcohol to remove the film of oil used to protect the tape. 

After calibration, a thin film of light oil, such as sewing machine oil, should be applied to 

the tape for protection. 

B.2. Tolerances 

 The tolerances for measuring tapes are those stated below. 

 Table B-1. Tolerances for a 30 meter tape. 

Length Interval Tolerances 

0 through 15 meter 1.27 mm (0.050 inch) 

15 through 22 meter 1.91 mm (0.075 inch) 

22 through 30 meter 2.54 mm (0.100 inch) 

B.2.1. The inaccuracy in the length of the ribbon, when supported on a horizontal 

surface with a tension of 10 pounds at a temperature of 20 °C (68 °F) shall not exceed 

0.050 inch for the 75 foot length, and 0.100 inch for the 100 foot length. 

B.3. Tension Specifications 

The length of a tape will be affected by the temperature of the tape, the tension applied to 

the tape, and the manner in which the tape is supported. The tape will stretch when 

tension is applied and will return to its normal length when the tension is removed, 

provided the tape has not been permanently deformed when it was stretched. The 

tensions, at which steel tapes are to be calibrated, expressed in terms of the load in 

kilograms (or pounds) to be applied to obtain the tension, are stated below. The loads 

should be accurate within 45 g (0.01 lb). 

Table B-2. Tension to be applied, in terms of load. 

Length Interval Tension 

Less than 10 m (25 ft) 2 kg (3.5 lb) 

10 m through 30 m (25 ft through 100 ft) 5 kg (10 lb) 

Greater than 30 m (100 ft) 10 kg (20 lb) 

B.4. Methods of Support and Tension Considerations 

Tapes calibrated in a State laboratory are normally supported on a horizontal surface 
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throughout the entire length of the tape. Also, tapes may be calibrated and used when 

supported in catenary types of suspension. In these cases, the tape is supported at 

equidistant points because the weight of the tape affects its length. The weight of the tape 

increases the tension and the “sag” causes the horizontal length to be shorter than when 

the tape is supported throughout its length. Equations are given in GMP No. 10 to 

compute the horizontal straight-line distance of a tape supported at N number of 

equidistant catenary suspensions and for computing the tension of accuracy, defined as 

the tension that must be applied to the tape interval to produce its designated nominal 

length at the observed temperature of the tape. 

It is sufficient to provide the user of a steel tape with the calibrated length of the tape 

under standard temperature and tension conditions, the weight per unit-length of the tape, 

and the AE value for the tape, as might be requested. This information will enable the 

user to compute the values desired using the equations cited above. 

B.5. Zero Reference Point 

Metal measuring tapes submitted to a State laboratory for calibration normally will be 

made of steel. Generally, these tapes will have a ring on the end of the tape. For 

maximum calibration and measurement accuracy, a tape should have a blank end 

between the ring and the zero graduation. The zero graduation is then more precisely 

defined and more easily referenced for calibration and use. 

Tapes that have the ring as part of the measuring portion of the tape are more difficult to 

calibrate than a tape with a blank end. When the ring is part of the measuring portion of 

the tape, the zero reference point shall be the outside end of the ring unless otherwise 

specified. It is more difficult to obtain a good zero reference setting on the ring due to its 

curvature and to parallax in reading the edge of the ring against a reference mark. 

Additionally, the ring may become permanently deformed in use and change the length of 

the tape. For these tapes the NIST normally calibrates from the 1 foot mark over the 

length of the tape, and then calibrates from the ring to the 1 foot mark. These values are 

reported separately so the user can obtain maximum measurement accuracy by using the 

1 foot graduation as the zero reference point. 

When the ring is part of the measuring range of the tape, a special holder for the ring is 

needed to clamp the tape to the length bench. A strap with an open area in the middle is 

needed to permit the end of the ring to be seen. The strap is slipped through the ring and 

the strap is clamped to the length bench. See example below. 

 Figure B-1. Strap, holder, and tape ring. 

 The edge of the tape to be calibrated (the reading edge) is the edge nearest the observer 
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when the zero graduation is to the observer’s left. When viewed through a microscope, 

some graduations will appear to have irregular edges. The portion of the graduation to be 

used for calibration is the portion of the graduation at the bottom of the reading edge of 

the tape. This provides a reference point that can be repeated and referenced by others. 

Do not attempt to estimate the ‘best overall’ edge of a graduation because this is not 

easily repeatable and cannot be accurately reproduced by other. If the graduations to be 

calibrated do not reach to the edge of the tape, the tape should not be calibrated. 

B.6. Temperature Considerations 

No temperature correction is required, provided the test tape and the standard tape are at 

the same temperature and of the same material. This will be the case when the 

measurements are made inside a building. Tapes of the same color would be expected to 

attain the same temperature, even in sunlight. However, black and white tapes have 

shown temperature differences of as much as 8 °C when exposed to direct sunlight. In 

such cases, the temperature differences, even when measured, would be uncertain due to 

variability of exposure along the length of the tape. Accordingly, calibrations in the 

laboratory are preferred, when possible. 

B.7. Invar tapes 

Invar is an alloy of nickel and steel. Invar tapes are used to obtain measurements of 

greater accuracy than can be made with steel tapes, because invar has a very low 

coefficient of expansion. It has the added benefit of being very slow to tarnish from 

exposure to the atmosphere. However, invar tapes require very careful handling to 

prevent twists and kinks. 

The load to be applied to an invar tape to maintain the desired tension is normally 20 lb. 

A load of 40 lb is used for the higher tension to determine the AE value (see C.8.). For 

metric tapes, the normal load is 5 kg. A load of 10 kg is used to determine the AE value. 

B.8. AE Value 

The AE Value (area elongation value) for a tape is determined by first calibrating the tape 

under its normal tension. The load is then increased by10 lb or 20 lb and one length 

interval is recalibrated to determine the length of the tape under the increased tension. 

The AE factor is computed with the following equation. 

Ln L L

Q Q AE

1 0

1 0



  (B-1) 

For example, a 100 foot tape is calibrated from 0 feet to 100 feet with a load of 10 lb 

applied to the tape with a resulting length of 99.992 feet. The load is increased to 20 lb 

and the new length is found to be 100.004 feet. The AE value is: 
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lb

feet

lb feet feet lb lb AE 83 333

0.012

1000 100 (100.004 99.992)(20 10 )    

It is recommended that the AE value be determined over the longest interval that is 

convenient to measure. This minimizes the error in the AE value because of the better 

readability of the change in length 

B.9 Weight per Unit Length 

The weight per unit length of a tape can be determined as follows: 

B.9.1. Weigh the tape and reel (or case). 

B.9.2. Remove the tape from the reel or case and weigh the empty reel (or case). 

B.9.3. Measure the length of any blank ends on the tape and add this to the measuring 

length. 

B.9.4. Correct for the weight of the loop on the tape. The weight of the loop that is 

normally used on steel tapes is approximately 2.5 grams. 

The weight per unit length is the computed as follows. 

length of tape length of blank ends

Weight of loaded reel weight of empty reel weight of loop Weight per Unit Length 

  