Scale calibration: scale classes, standards, test weights and intervals

Scale calibration means placing weights of known, traceable mass on a scale or balance and recording how far its readings are from those masses: at several loads across its range (errors of indication), repeatedly at one load (repeatability) and at different positions on the pan (eccentricity), with the uncertainty of each result (EURAMET cg-18, §4.1 and §5). It does not change the scale (VIM 2.39). Pressing the CAL button, or letting a balance adjust itself with its built-in weight, is an adjustment at one load (VIM 3.11): useful, but it says nothing about the rest of the range.

This page is the overview. For the CAL button, the three tests step by step, worked examples and USP <41>, see how to calibrate a scale.

Scale classes and the test weights they need

US and international rules group scales into four accuracy classes, and NIST Handbook 105-1 gives the weight classes that are likely to be suitable for each (HB 105-1, Table 1):

Scale class (Handbook 44 / OIML R 76)	Typical examples	ASTM E617 weights	OIML R 111 weights
I (special)	Analytical and micro balances	1	F1
II (high)	Precision balances, jewelry scales	1, 2	F1, F2
III (medium)	Bench, counting, platform and retail scales	3 to 6	M1, M2, M1–2, M2–3
IIII (ordinary)	Coarse industrial scales	3 to 6	M3

The rule behind the table: a weight’s error, plus its uncertainty if you use it at its nominal value, should stay below one third of the tolerance at the load you test (Handbook 44, App. A 3.2; OIML R 76-1, 3.7.1). Check the actual numbers at your loads; the procedure has two worked examples.

Standards and guides that apply

Document	Scope	Access
EURAMET cg-18 (v4.0)	Calibration of non-automatic weighing instruments: tests, uncertainty, certificate, minimum weight	Free
NIST Handbook 44 (2026)	US legal requirements and tolerances for commercial scales, adopted by all 50 states (NIST OWM, Weighing FAQs)	Free
NIST Handbook 105-1	Field standard weights, and which weight classes suit which scale class	Free
OIML R 76-1:2006	International requirements for non-automatic weighing instruments	Free
OIML R 111-1:2004	Weight classes E1 to M3 and their maximum permissible errors	Free
ASTM E617-23	US classes for laboratory weights and precision mass standards	Paid
USP <41> and <1251>	Balances in pharmaceutical work; revised chapters official since February 1, 2026	Paid

Calibration is not legal verification. A scale used to sell or charge by weight is a commercial device: it should have an active NTEP Certificate of Conformance and must be approved by the state where it is used (NIST OWM, Weighing FAQs). The procedure explains how the two differ.

Balance measurement uncertainty

EURAMET cg-18 splits the uncertainty of each error into the uncertainty of the indication (repeatability, display resolution, eccentricity) and the uncertainty of the reference mass (the weight certificates, their drift, air buoyancy and convection) (cg-18, §7.1). It keeps that uncertainty at calibration separate from the larger uncertainty of an everyday weighing, which adds temperature changes, drift between calibrations and how the balance is used (cg-18, §7.4). The relative uncertainty grows as the load gets smaller, which is where the minimum weight comes from: the smallest net sample whose relative uncertainty still meets your requirement (cg-18, App. G). Build a budget with the uncertainty calculator.

How often to calibrate a scale

EURAMET cg-18, OIML R 76 and Handbook 44 set no calibration interval for scales used outside legal metrology. Base it on risk, use and history, the risk-based approach the 2026 revision of USP <41> also asks for (ECA Academy, 2024), and recalibrate after the scale is moved: a certificate shouldn’t be accepted for a scale moved after calibration unless it is shown to be immune to the move (cg-18, §4.1.2). The weights need calibrating too: for field standard weights, NIST notes intervals often start at one year and are extended based on stability data, generally not beyond five years (HB 105-1, §6.4). See calibration intervals.

FAQ

Why does an analytical balance have measurement uncertainty?

Because every part of a weighing varies a little: the balance does not repeat exactly, its display rounds, the reading depends on where the load sits on the pan, and the test weights have their own uncertainty, drift, and an air buoyancy that depends on the day's air density. EURAMET cg-18 combines these into the uncertainty of each calibration result. Everyday weighings add more, such as temperature changes and drift between calibrations, which is why the in-use uncertainty is larger than the calibration uncertainty.

What does a NIST-traceable scale mean?

It means the scale was calibrated with test weights whose own calibration links to the SI, usually through NIST, by a documented chain of calibrations with stated uncertainties. The scale itself is not traceable; its calibration results are. Look for a certificate with the errors at several loads, the repeatability and eccentricity results and the uncertainty. NIST does not certify traceability claims made by others.

Can the same scale be used for trade and for lab work?

A scale used to sell or charge by weight in the US must meet NIST Handbook 44, should have an active NTEP Certificate of Conformance, and must be inspected and approved by the state where it is used. That legal approval is not a calibration with uncertainty, and a calibration is not legal approval. A lab or production scale needs a calibration against your own tolerance; a trade scale needs both if you also use it for quality decisions.