Standard weight classes: materials, tolerances and choosing the right weight

1. What does the class mean?

This guide uses OIML R 111-1:2004 together with Amendment 2025. Its nine classes are E1, E2, F1, F2, M1, M1–2, M2, M2–3 and M3. The intermediate M classes apply to nominal masses of 50–5 000 kg. A class includes physical requirements as well as mass limits; ASTM uses a different classification system.

References: OIML R 111-1:2004 · §§1.3, 5, 8–11, Table 1 · OIML R 111-1 · Amendment 2025

2. Material does not establish the class

Stainless steel, other alloys, brass, plated metals and cast iron are encountered depending on mass, shape and class. Class compliance also depends on corrosion resistance, density, magnetic properties, construction and surface condition.

References: OIML R 111-1:2004 · §§1.3, 5, 8–11, Table 1 · METTLER TOLEDO · Test weights and selection guidance

3. Maximum permissible errors depend on nominal mass

Selected Table 1 limits below apply to conventional mass. All numerical values mean ± mg, not percentages or uncertainty. For example, 1 kg F1 has an MPE of ±5 mg (±0.005 g), while 1 kg M1 has ±50 mg (±0.05 g). These are class limits, not the measured error of each weight.

References: OIML R 111-1:2004 · §§1.3, 5, 8–11, Table 1

4. MPE and uncertainty are different

MPE is an acceptance limit. Calibration estimates conventional mass, deviation and expanded uncertainty U. Read U from the certificate rather than substituting an MPE value.

The mass conditions in R 111-1 §5 include U at k = 2 ≤ MPE/3 and |mc − m0| + U ≤ MPE. Physical requirements also apply. For an illustrative 1 kg F1 weight, a +3 mg deviation with U = 1 mg gives 4 mg, satisfying this mass condition; +4.5 mg with the same U gives 5.5 mg and does not.

References: OIML R 111-1:2004 · §§1.3, 5, 8–11, Table 1

5. Start with the application's tolerance

Define the test load and permissible process error. Assess the reference weight's calibrated value, uncertainty and drift alongside balance performance. Readability alone cannot determine the class. E2/F1 may be candidates for demanding laboratory work; F2/M classes may suit production weighing when the uncertainty and acceptance criteria allow. Assess combined weights as a combination, not by one member's limit.

• Distinguish routine checks, calibration and adjustment
• Provide capacity, readability d and verification interval e where relevant
• Choose loads representative of use
• Define acceptance criteria and certificate data needed
• Review environment and historical performance

References: METTLER TOLEDO · Test weights and selection guidance

6. Handling and care

Use appropriate clean gloves, tweezers or handling tools. Keep weights in separate compartments, protect surfaces and allow thermal equilibration. Oils, dust, moisture, corrosion and wear may change mass. Required waiting time depends on the load and temperature difference.

Do not polish, repaint, label, engrave or open adjustment cavities yourself. Agree cleaning and any as-found requirements with the laboratory. Isolate dropped, corroded or suspect weights for assessment.

• E classes: control contamination, temperature, magnetism and buoyancy as needed
• F classes: preserve surfaces and calibration history
• M classes: inspect rust, coating loss, debris and contact bases
• Heavy weights: use suitable rated lifting equipment
• Set recalibration intervals using risk and historical results

References: NIST · Good Measurement Practices · METTLER TOLEDO · Test weights and selection guidance

7. Ask SK to assess your calibration requirements

Send photos, class, nominal values, quantities, identification, existing certificates and balance details. SK can assess the requested work and reporting needs. Confirm the applicable class, mass range, method and uncertainty against current accredited coverage before ordering an accredited service. Balance calibration and weight calibration are separate activities.