Calibration Weights Buyer's Guide: Choosing the ASTM or OIML Class for Your Balance

Calibration weights are graded by maximum permissible error, an absolute number of milligrams that depends on the class and on the nominal value, so the right class is the one whose tolerance is small next to your balance's readability. A 30 g ASTM Class 4 weight is good to ±0.9 mg, which is about one count on a 1 mg balance but nine counts on a 0.1 mg analytical balance.

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What do ASTM and OIML weight classes actually measure?

They measure how far a weight's true mass may sit from its printed nominal value. ASTM E617 is the North American specification and defines ten designations: 000, 00, 0, 1, 2, 3, 4, 5, 6 and 7. OIML R 111 is the international recommendation and defines nine: E1, E2, F1, F2, M1, M1-2, M2, M2-3 and M3. In both systems a higher number or a later letter means a looser tolerance.

OIML states the limit as an absolute mass for each nominal value and says it must not be extrapolated to values that are not listed, so read the tolerance for your exact denomination instead of scaling from a neighbour. Neither standard declares one class equal to a class in the other system. METTLER TOLEDO's class guide puts OIML E2 near ASTM Class 1 as the safest anchor and warns that vendor mapping tables disagree on F1 and F2. Pick the system your market or regulator names, then pick the class inside it.

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How tight is each class at 30 g, 100 g and 1 kg?

ASTM Class 1 is about 12 times tighter than Class 4 at 30 g and 8 times tighter at 100 g and 1 kg. The table reads the ASTM E617-13 metric tolerances as printed in A&D's weight selection guide, with OIML R 111 values from METTLER TOLEDO's table alongside. Confirm against the current edition of the standard before you write a tolerance into an SOP.

Nominal value	ASTM Class 1	ASTM Class 2	ASTM Class 3	ASTM Class 4	OIML E2	OIML F1
30 g	±0.074 mg	±0.15 mg	±0.45 mg	±0.9 mg	not listed	not listed
100 g	±0.25 mg	±0.5 mg	±1 mg	±2 mg	±0.16 mg	±0.5 mg
1 kg	±2.5 mg	±5 mg	±10 mg	±20 mg	±1.6 mg	±5 mg

Read as a share of the load, a 30 g Class 4 weight is within ±0.003% (0.9 mg of 30,000 mg). That looks small, and for a bench balance it is. The question is how it compares with the balance's own step.

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How do you match a class to your balance's readability?

Compare the weight's tolerance with one count of the balance. METTLER TOLEDO is explicit that no strict one-to-one rule links readability to class, so treat the arithmetic below as a screening check, not a specification.

• 10 mg balance: a ±0.9 mg Class 4 weight is under 0.1 of a count. Class 4 is more than enough.
• 1 mg balance: ±0.9 mg is about one count. Class 4 works for a routine check, and Class 3 (±0.45 mg) gives margin.
• 0.1 mg analytical balance: ±0.9 mg is nine counts. A reading that lands 0.5 mg off could be the balance or the weight, so the check cannot resolve the last digit. Class 1 (±0.074 mg, under one count) or Class 2 (±0.15 mg) can.

The same arithmetic applies to a pharmacopoeial accuracy test. Our laboratory balance guide covers the USP <41> rule: a balance passes if it reads within 0.10% of a test weight between 5% and 100% of capacity, and that weight's own maximum permissible error may be no more than one third of the test limit. At 30 g the limit is 30 mg, one third is 10 mg, and a Class 4 weight's 0.9 mg uses under a tenth of that allowance. The 5% floor means a 30 g weight suits balances up to 600 g capacity.

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How do you choose a calibration weight in four steps?

Work from the balance and the paperwork, not from the weight catalogue.

1. Write down the balance's readability and capacity. The readability sets the tolerance you can use; the capacity sets the load, since a check should sit between roughly 5% and 100% of capacity.
2. Name the standard your method or auditor cites. ASTM E617 or OIML R 111. If the SOP only says "traceable weights", ask which, in writing.
3. Choose the class whose tolerance is about one count or less at your test load. Use the table above and the tolerance for the exact denomination.
4. Choose the certificate. Decide between an ISO/IEC 17025 accredited certificate and a NIST-traceable one, covered next.

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Which certificate do you need: accredited or traceable?

It depends on whether your quality system must see the weight's uncertainty. An ISO/IEC 17025 accredited certificate comes from a laboratory assessed for competence and states a measurement uncertainty for each weight. A traceable certificate links the weight to national standards without that accreditation. A&D notes that certified weights carry longer lead times because they may be sent out for certification.

Under OIML R 111, the calibration's expanded uncertainty (coverage factor k = 2) must not exceed one third of the class tolerance. As a yardstick, the 30 g Class 4 weight listed below carries 0.18 mg against a 0.9 mg tolerance: one fifth, inside that one-third limit. Legal verification for trade is a different process and does not replace an accredited calibration report.

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Which calibration weight does ERE list today?

ERE lists the OHAUS ASTM Individual Calibration Weights, a line described as adjusted to meet or exceed ASTM Class 1 to 4 tolerances. The listed model is part number OH-30391314, a 30 g Class 4 weight with an ISO/IEC 17025 accredited certificate, two-piece construction, a cylindrical body with handling knob and a protective case, with a stated uncertainty of 0.18 mg. The product record describes stainless-steel construction, including a controlled-density alloy, and certificate options traceable to NIST. Class 1 is described as the primary reference for calibrating other mass standards and for balances with readability as fine as 0.1 mg.

Only the 30 g Class 4 model is listed online. If your balance needs a different denomination or a tighter class, send the balance model, readability, test load and the standard your method cites, and we will quote the matching weight. See the Calibration Weights collection and the Scales & Balances collection. Pricing is by quote.

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When are M-class and trade weights the right call?

For platform and floor scales, not for laboratory balances. M1, M2 and M3 (OIML) and the looser ASTM classes, such as Class 6 and 7, are the working grades for commercial and industrial weighing. Where the weighing sets the quantity sold, Canadian trade rules apply; our industrial scale guide covers the Measurement Canada side. Heavy cast-iron field weights suit that job; a stainless knob weight does not.

Frequently Asked Questions

What is the difference between ASTM and OIML calibration weight classes?

ASTM E617 is the North American system, with designations from 000 to 7, and OIML R 111 is the international one, with classes from E1 to M3. Both state tolerance as an absolute mass for each nominal value, but neither equates a class in one system to a class in the other, so any mapping is approximate.

Which ASTM class do I need for an analytical balance?

For a 0.1 mg balance, a Class 1 or Class 2 weight is the practical choice: at 30 g their tolerances are ±0.074 mg and ±0.15 mg, which is about one count or less. A Class 4 weight is ±0.9 mg at 30 g, nine counts on that balance.

How often should calibration weights be recalibrated?

There is no single interval. It depends on how often the weights are used, the environment, the handling and your quality system. Weights handled without gloves or forceps should be rechecked more often.

Do I need an ISO/IEC 17025 certificate for my weights?

Only if your quality system requires the weight's measurement uncertainty on record, as GMP, GLP and pharmacopoeial work often does. For routine checks a NIST-traceable certificate may be enough; confirm what your auditor or method requires.

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Lire en français : Poids d'étalonnage : quelle classe ASTM ou OIML choisir pour votre balance