How often should a scale be calibrated?

No standard sets one interval. Base it on how much and how hard the scale is used, its environment, the risk of an error and its own as-found history, as ILAC-G24 recommends. Check it with a check weight before use or daily between calibrations, and recalibrate after it is moved or repaired.

How do you calibrate a weighing scale?

Calibrate it where it is used, with certified test weights at room temperature. Load the same weight at least five times for repeatability, move a load of about one-third of capacity from the center to four positions near the corners, and apply at least five loads from zero to full capacity. Record each indication, work out the errors and their uncertainty, and compare them with the limit for the scale's class or your own tolerance.

What weights do I need to calibrate a scale?

Weights with a current calibration certificate whose error is no more than one-third of the scale's maximum permissible error at each load. OIML R 111 grades weights from class E1 to M3. Class M1 weights are intended for medium accuracy (class III) scales, and F1 or F2 weights for high accuracy (class II) balances.

What is the eccentricity or corner load test?

It checks whether a scale reads the same when the load sits off center. A load of about one-third of capacity is placed in the center and then near each corner, and each indication must stay within the maximum permissible error for that load. NIST Handbook 44 calls it the shift test.

What do d and e mean on a scale?

d is the actual scale interval, the step the display shows. e is the verification scale interval that the accuracy class and its error limits are based on. On class III and IIII scales the two are normally equal, while some class I and II balances show a finer d than e.

Compliance and audits

Scale calibration: procedure, test weights and how often to calibrate

13 min read Published November 8, 2026 Updated November 9, 2026

Scale calibration puts test weights of known mass on a scale or balance to measure how well it repeats, its error at five or more loads across its range and its error when the load sits off center (the eccentricity or corner load test), and states each error with its uncertainty. A scale used for trade also needs official verification against legal tolerances, under NIST Handbook 44 in the US and under national rules elsewhere, for which OIML R 76 is the international model.

On this page

What a scale calibration checks

A calibration applies test loads, records the indication for each and works out the error and its uncertainty. EURAMET Calibration Guide No. 18 (cg-18, version 4.0, 2015) is the European guideline for calibrating non-automatic weighing instruments. It describes three tests: repeatability, errors of indication across the range and eccentricity. It covers self-indicating instruments, and its worked examples run from a 220 g balance to a 30,000 kg scale.

cg-18 sets no error limits of its own. A pass or fail needs a tolerance from the scale's accuracy class, the maker's specification or your process, and cg-18 shows how to state conformity with that tolerance once the uncertainty is taken into account.

Calibration is normally done where the scale is used. Local gravity, the environment and transport all affect a scale, so cg-18 warns that moving it after calibration may invalidate the result.

Calibration vs verification

Accuracy classes and the scale interval

OIML R 76-1:2006, the international recommendation for non-automatic weighing instruments, sorts them into four accuracy classes: special (I), high (II), medium (III) and ordinary (IIII). The class depends on the verification scale interval e and on the number of intervals n, which is the maximum capacity Max divided by e. A class III scale with e of 5 g or more, for example, has between 500 and 10,000 intervals. The load cells inside a scale have accuracy classes of their own, A to D, under OIML R 60.

The actual scale interval d is the step the display shows. The verification scale interval e is the value the class and its limits are based on. Only class I and II instruments may have an auxiliary indicating device that shows a finer step than e, so on class III and IIII scales e and d are normally equal.

NIST Handbook 44, the US code for commercial weighing and measuring devices, uses the same idea with classes I, II, III, III L and IIII. Its Table 7a gives the typical class for each use: class I for precision laboratory weighing, class II for laboratory weighing and precious metals, class III for most other commercial weighing, and class III L for vehicle, livestock, railway track, crane and hopper scales other than grain hoppers.

Load cell calibration

Maximum permissible errors by class

R 76 expresses each limit in multiples of e, and the limit grows with the load. The table gives the maximum permissible errors on initial verification. For instruments in service, R 76 allows twice these values.

OIML R 76-1:2006 Table 6: maximum permissible errors on initial verification, for a load m expressed in verification scale intervals e
Class	±0.5 e	±1.0 e	±1.5 e
I	0 ≤ m ≤ 50,000 e	50,000 e < m ≤ 200,000 e	m > 200,000 e
II	0 ≤ m ≤ 5,000 e	5,000 e < m ≤ 20,000 e	20,000 e < m ≤ 100,000 e
III	0 ≤ m ≤ 500 e	500 e < m ≤ 2,000 e	2,000 e < m ≤ 10,000 e
IIII	0 ≤ m ≤ 50 e	50 e < m ≤ 200 e	200 e < m ≤ 1,000 e

Tip: A worked example. Take a class III platform scale with Max = 30 kg and e = 10 g, so n = 3,000. On initial verification the limit is ±5 g up to 5 kg (500 e), ±10 g up to 20 kg (2,000 e) and ±15 g from there to 30 kg. In service each limit doubles, to ±10 g, ±20 g and ±30 g.

Handbook 44 tolerances for US trade scales

R 76 starts from the initial limits and doubles them in service. Handbook 44 starts from the other end. Its Table 6 gives maintenance tolerances, which apply to equipment in use, and acceptance tolerances are half of them. They apply to new equipment, to its first official test within 30 days of going into service, and to the first official test within 30 days after a rejection is corrected or after a major overhaul.

For the 30 kg example, HB 44's class III acceptance tolerances come out equal to the R 76 initial limits, and its maintenance tolerances equal the R 76 in-service limits. The two schemes do not match at every load and class, so a US trade scale is judged by HB 44 as your state has adopted it.

Test weights: OIML R 111 classes and the one-third rule

OIML R 111-1:2004 grades weights into classes E1, E2, F1, F2, M1, M1-2, M2, M2-3 and M3, from the most to the least accurate. Each class has a maximum permissible error for each nominal value. R 111 also requires the expanded uncertainty of a weight's calibration (k = 2) to be no more than one-third of that error. Classes M1-2 and M2-3 are for weights from 50 kg to 5,000 kg used with class III scales.

The weights have to be good enough for the scale. R 76 clause 3.7.1 says the error of the standard weights may not exceed one-third of the scale's maximum permissible error for the applied load. In the 30 kg example the initial limit at full load is ±15 g, so the 30 kg of weights may be off by no more than 5 g. Class M1 weights qualify: R 111 allows a 10 kg M1 weight ±500 mg and a 20 kg one ±1 g.

US weights and measures work uses the same rule. HB 44 Appendix A says a standard used without correction must have a combined error and uncertainty below one-third of the device tolerance. NIST Handbook 105-1 (2019) has officials choose field standard weights from ASTM E617 or OIML R 111.

OIML R 111-1:2004 weight classes: maximum permissible error at two nominal values (Table 1) and the instruments each class is intended for (clause 1.3.1)
Class	1 kg weight	10 kg weight	Intended use with scales
E2	±1.6 mg	±16 mg	Special accuracy class I instruments
F1	±5.0 mg	±50 mg	Class I and class II instruments
F2	±16 mg	±160 mg	Class II instruments, for example for precious metals
M1	±50 mg	±500 mg	Class III instruments
M2	±160 mg	±1,600 mg	Class III instruments and general commercial use
M3	±500 mg	±5,000 mg	Class III and class IIII instruments

Metrological traceability explained

Tip: A weight set used as a reference needs its own calibration certificate, traceable to national standards. R 111 requires one for E1 and E2 weights. For the other classes the certificate is still what carries the traceability, so ask your lab for one.

The repeatability test

Put the same load on the scale several times under the same conditions and record each indication. cg-18 asks for at least 5 loadings, or at least 3 where the load is 100 kg or more. The load should be a single body if possible. It does not need to be a calibrated weight unless its results also count toward the errors of indication. A load between half and full capacity is common.

Set the indication to zero before you start. After each removal, check the zero and reset it if needed. Record whether zero tracking is on. The result is the standard deviation of the repeated indications of that one load.

For verification, R 76 asks for one series at about 0.8 Max: three weighings on class III and IIII scales, six on class I and II. Under clause 3.6.1 the results may differ by no more than the maximum permissible error for that load. HB 44 T.N.5 sets the same rule with the maintenance tolerance.

Type A uncertainty calculator

Errors of indication across the range

This is the linearity check. cg-18 asks for at least five test loads spread fairly evenly over the range, for example zero or Min, a quarter, half and three quarters of Max, and Max. Each error is the indication minus the conventional mass of the test load.

R 76 adds that the test loads should include Max and the loads where the maximum permissible error changes, which are 500 e and 2,000 e on a class III scale. Choose the loading pattern from how the scale is used. cg-18 describes unloading between steps, building up in steps without unloading, and stepping up then back down.

On large scales the test load can be built up by substitution. The standard weights are applied and their indication noted. They are then replaced with any other stable load until the scale shows the same indication, and the weights go back on top. For verification, R 76 asks for standard weights of at least half of Max when substitution is used, or less if the scale repeats well.

Record the indications as read, not the errors. cg-18 asks for the original indications so the errors can be worked out and checked later.

The eccentricity test (corner load test)

Place one test load in the center of the platform. Then move it to four positions near the corners: front left, back left, back right and front right. Record the indication at each position. Each indication must stay within the maximum permissible error for that load (R 76 clause 3.6.2). cg-18 suggests a load of about one-third of Max. R 76 uses one-third of the sum of Max and any additive tare, applied to the four quarter segments in turn on a scale with up to four points of support. R 76 has automatic zero-setting and zero tracking switched off for the test.

A scale with more than four points of support is loaded over each support instead. R 76 applies 1/(n − 1) of the sum of Max and additive tare at each of the n supports.

Handbook 44 calls this the shift test. For a scale of 500 kg (1,000 lb) capacity or less it uses a test load of one-third of capacity, between 30% and 35%, centered in each quadrant. A larger scale may use that pattern or a load of one-quarter of capacity over each corner.

Conditions on site

Calibrate under the scale's normal conditions of use. Drafts, vibration and the tilt of the platform then show up in the results and in their uncertainty. Measure the air temperature near the scale at least once during the calibration.

Let the weights reach the temperature of the room. A weight warmer or colder than the air sets up air currents that change the indication, and the effect grows with the size of the weight. cg-18 notes that it can be significant for high-accuracy weights such as classes E2 and F1.

As-found and as-left results

Run the tests before anyone adjusts the scale. Those are the as-found results, and they show how the scale weighed since its last calibration. If it is then adjusted, repeat the tests and record the as-left results. cg-18 asks the certificate to state any adjustment and, for an adjustment with an external weight, which weight was used.

When an as-found error is outside the limit, ISO 9001 clause 7.1.5.2 asks you to determine whether earlier results were adversely affected and to act where needed. For a scale that means the batches or test results it weighed since its last good calibration. The sign of the error matters. When you fill to a target weight, a scale that reads high leaves packages underfilled, and one that reads low overfills them.

Calibration vs verification of legal-for-trade scales

A scale used to sell by weight falls under weights and measures law. In the US, NIST Handbook 44 holds the specifications and tolerances adopted by the National Council on Weights and Measures, which recommends them to the states for their control of commercial devices. Outside the US, national legal metrology rules apply. OIML expects its member states to implement R 76 as far as possible, and Europe uses EN 45501, which cg-18 cites alongside R 76.

Verification in this sense is an official test against legal tolerances, and a scale that passes may carry a verification mark or seal. It decides whether the scale may be used for trade. Calibration reports the scale's errors and their uncertainty for your own measurements. A scale used for trade and for quality decisions may need both.

HB 44 General Code G-S.8 requires a seal that must be broken, or an approved audit trail, before anyone changes an electronic adjustment that affects accuracy. Check with your weights and measures office before a sealed scale is adjusted.

HB 44 can also serve scales that are not used for trade. Its introduction notes that it may be cited as the standard in a quality system or a contract.

Calibration vs verification

How often to calibrate a scale

No standard sets one interval for all scales. ILAC-G24 / OIML D 10 (2022 edition) lists the factors to weigh, including the uncertainty you need, the risk of exceeding the limits in use, the maker's recommendation, wear and drift, how much and how hard the scale is used, its environment, transport and the results of intermediate checks. It also advises calibrating new equipment more often at first.

Between calibrations, check the scale with a check weight before use or once a day. Put one load near the range you use on the center of the platform and compare the indication with the value found at the last calibration. ISO/IEC 17025:2017 clause 6.4.10 asks for intermediate checks where they are needed to keep confidence in the equipment. A log of those checks is also the evidence for a longer interval later.

Recalibrate after the scale is moved, repaired or overloaded. A failed check, or a drift toward the limit, is a reason to shorten the interval.

What the certificate and record should show

EURAMET cg-18 lists what a scale calibration certificate should give, in line with ISO/IEC 17025. The essentials are below.

The scale's ID, maker, type, Max, d and place of installation.
The date, the site and the conditions during the calibration, and any adjustment made.
The test weights used, with their class and certificate numbers, for traceability.
The indication and error at each test load, as found and, after any adjustment, as left.
The standard deviation from the repeatability test and the results of the eccentricity test.
The expanded uncertainty of each error, with its coverage factor k.
Any statement of conformity, the tolerance it was judged against and the decision rule.

How to calibrate a scale, step by step

Check that the test weights have current certificates and that their error is no more than one-third of the scale's limit at each load.
Let the weights reach room temperature next to the scale. Record the air temperature and whether zero tracking is on.
Inspect the scale. The platform should move freely without touching anything, and the display should return to zero.
Run the repeatability test: the same load at least 5 times (3 times at 100 kg or more), checking the zero between loadings.
Run the eccentricity test with about one-third of Max, in the center and at four positions near the corners.
Apply at least five test loads from zero to Max, including the loads where the limit changes, and record each indication as found.
Compare each error with the limit for that load, from the accuracy class, the maker's specification or your process tolerance, using your documented decision rule.
If the scale needs adjustment, adjust it, with the authority's agreement for a sealed trade scale. Then repeat the tests and record the as-left results.
Record the results, the weights used, the uncertainty and the next due date, and label the scale with its status.

How Axiospec helps with scale records

Axiospec is calibration management software. Each scale gets a record with its calibrations, its certificates and its next due date. A calibration can carry test points, each with its nominal load, its tolerance and the as-found and as-left readings.

Register your test weight set as a reference standard and pick it on each calibration. The set's own record then lists the calibrations it was used on, so a set found out of tolerance points you to the scales it checked. When a calibration is logged as Fail, the scale goes into quarantine and the record asks for the out-of-tolerance impact. Axiospec does not test the scale or make it legal for trade. On an in-house calibration, a final reading outside its tolerance sets the result to Fail, and recording a pass anyway takes a written reason. The tolerance, the decision rule and the interval stay with you or your lab. Legal approval stays with the weights and measures authority.

It is free for up to 50 active instruments. A new workspace starts with a 45-day trial of Professional, with room for 1,500 active instruments. We load your existing records for you at no charge on every plan.

Common questions

How often should a scale be calibrated?: No standard sets one interval. Base it on how much and how hard the scale is used, its environment, the risk of an error and its own as-found history, as ILAC-G24 recommends. Check it with a check weight before use or daily between calibrations, and recalibrate after it is moved or repaired.
How do you calibrate a weighing scale?: Calibrate it where it is used, with certified test weights at room temperature. Load the same weight at least five times for repeatability, move a load of about one-third of capacity from the center to four positions near the corners, and apply at least five loads from zero to full capacity. Record each indication, work out the errors and their uncertainty, and compare them with the limit for the scale's class or your own tolerance.
What weights do I need to calibrate a scale?: Weights with a current calibration certificate whose error is no more than one-third of the scale's maximum permissible error at each load. OIML R 111 grades weights from class E1 to M3. Class M1 weights are intended for medium accuracy (class III) scales, and F1 or F2 weights for high accuracy (class II) balances.
What is the eccentricity or corner load test?: It checks whether a scale reads the same when the load sits off center. A load of about one-third of capacity is placed in the center and then near each corner, and each indication must stay within the maximum permissible error for that load. NIST Handbook 44 calls it the shift test.
What is the difference between calibrating and verifying a legal-for-trade scale?: Verification is an official test against legal tolerances, under NIST Handbook 44 in the US or national rules elsewhere, and it decides whether the scale may be used for trade. Calibration measures the scale's errors and their uncertainty for your own measurements. A scale used for trade and for quality decisions may need both.
What do d and e mean on a scale?: d is the actual scale interval, the step the display shows. e is the verification scale interval that the accuracy class and its error limits are based on. On class III and IIII scales the two are normally equal, while some class I and II balances show a finer d than e.

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