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GMP 11

Good Measurement Practice

for

Assignment and Adjustment of Calibration Intervals for Laboratory Standards

1 Introduction

Purpose

Measurement processes are dynamic systems and often deteriorate with time or use.

The design of a calibration program is incomplete without some established means 

of determining how often to calibrate instruments and standards. A calibration 

performed only once establishes a one-time reference of uncertainty. Periodic 

recalibration detects uncertainty growth, serves to reset values while keeping a 

bound on the limits of errors and minimizes the risk of producing poor measurement 

results. A properly selected interval assures that an item will be recalibrated at the 

proper time. Proper calibration intervals allow specified confidence intervals to 

be selected and they support evidence of metrological traceability. The 

following practice establishes calibration intervals for standards and 

instrumentation used in measurement processes.

Note: This Good Measurement Practice provides a baseline for documenting 

calibration intervals. This GMP is a template that must be modified beyond Section 

4.1 to match the scope1 and specific measurement parameters and applications in 

each laboratory. Legal requirements for calibration intervals may be used to 

supplement this procedure but are generally established as a maximum limit 

assuming no evidence of problematic data.

For legal metrology (weights and measures) laboratories, no calibration interval 

may exceed 10 years without exceptional analysis of laboratory measurement 

assurance data. Associated analyses may include a detailed technical and statistical 

assessment of historical calibration data, control charts, check standards, internal 

verification assessments, demonstration of ongoing stability through multiple 

proficiency tests, and/or other analyses to unquestionably demonstrate adequate 

stability of the standards for the prescribed interval. Extended calibration intervals

in legal metrology laboratories recognized by the NIST Office of Weights and 

Measures, must approved by NIST OWM. (See also GLP 4). 

 1The laboratory scope is also called calibration and measurement capability (CMC). The CMC includes 

1) measurement parameters, 2) range of nominal values, 3) associated expanded uncertainties, and 4) documented 

and validated calibration methods. 
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Prerequisites

Calibration history and valid calibration certificates, with suitable 

calibration providers and sufficiently small uncertainties, for all laboratory 

standards.

Up-to-date measurement assurance and uncertainty data, with adequate 

degrees of freedom, for all measurement processes in the laboratory that 

match the laboratory Scope.

Expected tolerance limits where applicable.

Safety

No outstanding safety concerns

2 Methodology

Summary

Recommended calibration intervals are based on various examples of metrological 

traceability as described in GMP 13, Good Measurement Practice for Ensuring 

Metrological Traceability. As data is collected and evaluated, the laboratory 

technical manager may adjust the calibration intervals to ensure that measurement 

results are not invalidated by the intervals selected according to this procedure.

Apparatus

None.

Procedure

Identification of Parameters

The laboratory must identify all measurement parameters associated with 

the Scope of calibrations (and tests) that will be performed in the laboratory.

All standards used in a calibration process must be assessed to determine 

their level of significance and to minimize the risk of producing erroneous 

results on calibration certificates. 

Standards are identified as being critical parameters or secondary 

parameters.

2.3.2.1 Critical Parameters

Components that contribute more than 25 % of a measurement’s 

uncertainty are identified as critical parameters with a higher risk for 

impacting the measurement results. To ensure an accurate 
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evaluation of performance, calibration intervals are determined to 

meet a 99 % reliability target.

2.3.2.2 Secondary Parameters

Components that contribute less than 25 % but more than 1 % of a 

measurement’s uncertainty are identified as secondary parameters. 

Secondary parameters are assigned calibration intervals designed to 

meet a 95 % reliability target.

Assignment of Initial Intervals

Assignment of initial intervals is based on examples and recommendations 

given in this GMP as maximum limits. Otherwise, an initial interval is 

equivalent to the amount of time that passes before a metrologist makes the 

first observation of a result lying outside the warning limits of a control 

chart when the investigation yields no apparent correctable cause or when 

results from proficiency tests fail the normalized error assessment (or when 

the observed bias exceeds the laboratory uncertainty), or the maximum 

authorized through law or technical review.

Initial Intervals – Additional Standards

A calibration program and periodic calibration interval must be documented 

for all standards used in the laboratory to comply with the definition and 

interpretation of metrological traceability. Statements such as “as needed” 

are not acceptable alone without additional qualifications. A statement such 

as “calibrations are conducted on an annual basis and may be extended 

based on demonstrated control through technical and statistical analysis of 

historical calibration data, control charts, surveillance testing, and multiple 

interlaboratory comparisons, but will not exceed five years” would be 

acceptable. 

Absence of Control Charts or Measurement Assurance Data

If no initial measurement assurance data is available, the laboratory’s 

Technical Manager should assign initial intervals based on guidance and 

limits provided in this GMP. Intervals may be adjusted based on adequate 

data and criteria in Section 2.4 and NIST recommendations. Lacking 

adequate data for setting and adjusting calibration intervals is a risk that 

must be addressed by the laboratory.

Tracking and Scheduling

The laboratory must have a program of documenting and monitoring 

calibration status and scheduling calibrations on the defined intervals.

Examples include software systems that flag past-due standards prior to use, 

scheduling on a calendaring system, or periodic reviews to monitor the 
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calibration status prior to regular management reviews so that adequate 

resources are available to obtain calibrations in a timely manner and to 

prevent the use of standards after their calibration due date(s). 

Adjustment of Intervals

Intervals may be adjusted when determined to be necessary by the 

laboratory’s Technical Manager based on adequate data, valid technical and 

statistical analysis, and documentation of the assessment. Subsequent 

intervals may be adjusted based on documented analysis of adequate 

measurement assurance data, demonstrated control through surveillance 

assessments, and ongoing stability as demonstrated through multiple 

interlaboratory comparisons or proficiency tests.

Calibration intervals may be lengthened or shortened by performing a 

technical and statistical analysis, taking the following factors into 

consideration as appropriate, and fully documenting the data and 

assessment:

• calibration history; 

• measurement assurance data;

• interlaboratory comparisons; 

• data for the population of similar standards, equipment or 

technologies;

• NIST recommendations;

• statistical analysis methods; and

• manufacturer′s recommendations.

Inadequate reasons for adjusting intervals might include failure to have an 

adequate system for tracking due dates, lack of funding for suitable 

calibrations, loss of competent staff, or other related operational risks.

Extending calibration intervals must be based on valid, technically 

supported, and documented assessments to ensure that metrological 

traceability is supported for all measurement results provided by the 

laboratory. 

3 Assignment of Uncertainty

The uncertainty associated with the establishment of calibration intervals is not included 

as a part of the uncertainties associated with the respective measurement SOPs. See SOP 

29 for calculating uncertainties for standards and measuring instruments.
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4 Calibration Intervals for the Laboratory Scope

As this document is a template, the laboratory may eliminate all examples in this 

section and reference another laboratory document that combines metrological 

traceability information and calibrations in one file if it completely states 

calibration intervals. It is NOT recommended to include calibration intervals in the 

laboratory Quality Manual to ensure that updates are regularly implemented when 

calibrations are performed without excessive document approval requirements. The 

tables of calibration intervals noted in this GMP may be combined with the tables 

of reference, working, and check standard tables from the worksheets and 

hierarchies of GMP 13 to minimize the need for multiple tracking methods in the 

laboratory.

In the following sections, where Lab or Accredited Lab is noted, the measurement 

capability must be noted on the Scope of the laboratory performing the 

measurements and suitable supplier evaluations must be in place. For each level of 

calibration, a higher-level calibration is required for check standards.

Examples for Mass

Mass Critical Parameters

4.3.1.1 Balance and Measurement Process Performance

It is important to keep measuring equipment clean and in good 

working condition. If problems with the equipment are observed, the 

device should be evaluated and may require maintenance or repair 

before being put back into service. Regular service, maintenance, 

and calibration schedules may prevent equipment problems from 

occurring. When problems are observed, immediate service is 

required prior to use. 

Balance performance in weighing processes following SOP 5 and 

SOP 28 (e.g., Echelon I) is evaluated in each measurement series.

An F-test ratio evaluates the observed standard deviation of the 

process against the accepted standard deviation of the process.

However, while balance calibration might be considered part of a 

mass calibration procedure, maintaining equipment in good 

operating condition that minimizes contamination and prevents 

deterioration is a requirement for all laboratory balances and 

comparators as a part of accreditation and recognition. The 

laboratory may have additional procedures to ensure suitable 

maintenance of balances that ensures proper functioning and 

prevents contamination and deterioration.
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All other weighing processes have incorporated measurement 

control procedures and control charts that must be evaluated for 

balance performance characteristics as data is collected.

Table 1. Recommended intervals for balances used for mass (and gravimetric volume) calibrations.

Equipment Initial Service and Calibration

Interval (months) Source

Balances and Mass 

Comparators 12 Approved Provider

4.3.1.2 Mass Standards

Each weighing series in Echelon I also incorporates a check standard 

with a value that is evaluated using a t-test. The observed value is 

compared to the accepted reference value to determine if there might 

be problems. Even with the extreme care in storage and handling 

required for mass standards at this level, mass standards are dynamic 

with use. Wear, contamination and other factors can cause drift from 

accepted values. Thus, the following intervals have been set:

Table 2. Recommended calibration intervals for mass standards where Echelon I or II are in place

and standards are used at Echelon I or II.

Standards Initial Interval (months) Source

R 1. kg and R 1. kg 48 NIST

Sc 1. kg and Sc 1. kg (alternating 2 years) 48 NIST

R 100. g and R 100. g 48 NIST

Sc 100. g and Sc 100. g (alternating 2 years) 48 NIST or Accredited Lab

R 30 kg to R 2 kg 12 Lab (Echelon I) or Accredited Lab 

(Echelon II)

R 500 g to R1 mg 12 Lab (Echelon I) or Accredited Lab 

(Echelon II)

Sc 500 g to Sc 1 mg 12 Accredited Lab 

W 25 kg to W 1 mg 12 Lab

R 500 lb to 1 μlb 12 Lab

W 5000 lb to 1 μlb 12 Lab

Sensitivity weights and tare 

weights used as standards

12 Lab

R = Reference; Sc = check/control; W = working standards

Note: where Echelon I is not in place, all reference standards or working standards may need to be 

calibrated by a suitable calibration provider.
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Table 3. Recommended calibration intervals for mass standards for Echelon III calibrations.

Standards Initial Interval (months) Source

W 25 kg to W 1 mg* 24 Accredited Lab

Sc 25 kg to Sc 1 mg* 24 Accredited Lab

R 500 lb 120 Accredited Lab

W 5000 lb to 500 lb 24 Lab or Accredited Lab

Sensitivity weights and tare 

weights used as standards

12 Lab or Accredited Lab

R = Reference; Sc = check/control; W = working standards

Note: where Echelon II is not also in place, reference standards and/or working standards may all need 

to be calibrated by a suitable calibration provider. *When cast iron working and check standards are 

used, the initial calibration interval is recommended to be 6 months until adequate measurement 

assurance data is collected showing artifact stability prior to extending the interval.

Mass Secondary Parameters 

Availability of multiple units for environmental equipment enable the 

laboratory to conduct internal comparisons immediately after a calibration 

or between calibration cycles and generate internal calibration reports to 

enable extension of calibration intervals if adequate stability is 

demonstrated. See procedure for adjusting calibration intervals. The 

following table contains initial calibration intervals. 

Table 4. Recommended calibration intervals for environmental standards used in calibrations.

Standards Initial Interval (months) Source

Barometer 12 Accredited Lab

Hygrometer 24 Accredited Lab

Thermometer (digital) 12 Accredited Lab

Data Loggers (when used for 

calibrations) 12 Accredited Lab

Examples for Length 

Length Critical Parameters

4.4.1.1 Length Standards 

Length standards are dynamic with use. Wear, contamination and 

other factors can cause drift from accepted values. The following 

intervals have been set due to these factors:
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Table 5. Recommended calibration intervals for length standards.

Standards Initial Interval (months) Source

100 ft Tape #1 60 NIST

100 ft Tape #2 60 NIST

25 ft or 7 m Tape 60 NIST

18 in Steel Rule 120 NIST

Length Bench 24 (if used or moved) Lab

Length Secondary Parameters

Table 6. Recommended calibration intervals for length secondary parameters used in calibrations.

Standards Initial Interval (months) Source

Thermometer (digital) 12 Accredited Lab

Examples for Volume

Volume Critical Parameters

4.5.1.1 Volume Standards 

Volume standards are dynamic with use. Wear, contamination and 

other factors can cause drift from accepted values. Seals and valves 

must regularly be assessed for leaks and stability. Initial calibration 

intervals are as follows:

Table 7. Recommended calibration intervals for volume standards.

Standards Initial Interval (months) Source

R 100 gal standard 60 NIST, Lab, or Accredited 

Lab

R 25 gal standard * 60 Lab or Accredited Lab

R 5 gal standard * 24 Lab or Accredited Lab

Glassware- Laboratory standard 

Autopipettes* 5 L to 100 mL 120 Lab or Accredited Lab

*Gravimetric calibration for volumes 5 gallon or smaller, and all “slicker plate” standards. Laboratory 

must be qualified for performing gravimetric calibrations. Volume transfer is acceptable above 5 gallons.

*May be a “slicker plate” type. None are hand-held, “dump” style, test measures.

This GMP does not cover calibration intervals for pipettes used in analytical laboratories. 
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Volume Secondary Parameters

Table 8. Recommended calibration intervals for volume secondary parameters (gravimetric) used in 

calibrations.

Standards Initial Interval (months) Source

Barometer 12 Accredited Lab

Hygrometer 24 Accredited Lab

Thermometer (digital) 12 Accredited Lab

Table 9. Recommended intervals for volume secondary parameters (transfer) used in calibrations.

Standards Initial Cal Interval (months) Source

Thermometer (digital) 12 Accredited Lab

Example for Thermometry

Thermometry Critical Parameters

4.6.1.1 Temperature Standards 

Temperature standards are dynamic with use. Shock, contamination 

and other factors can cause drift from accepted values. Recalibration 

intervals are as follows:

Table 10. Recommended intervals for temperature standards.

Standards Initial Cal Interval (months) Source

25.5 ohm SPRT 36 NIST

100 ohm PRT′s 12 Accredited Lab

Standard Thermistor 12 Accredited Lab

Check Standards 12 Accredited Lab

Liquid-in-glass standards* 6* Accredited Lab

*Annual inspection must also ensure that there is no damage or separation in the liquid column. See 

NIST SP-1088 (2009)2 for additional maintenance plan requirements. New thermometers should be 

checked at least once a month at the ice point for a minimum of the first six months of use.

Where internal intrinsic or reference standards are available, and the laboratory has the 

demonstrated competency, the following may be used to set or adjust calibration intervals: triple 

point cells, melting point cells, and ice baths (using documented and validated procedures).

 2 NIST SP 1088, Maintenance and Validation of Liquid-in-Glass Thermometers, Christina D. Cross, Dean C. Ripple, 

W. W. Miller, Gregory F. Strouse, January 01, 2009, http://www.nist.gov/manuscript-publicationsearch.cfm?pub_id=900914.
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