Mass, Force & Torque · 4 min read
Mass Standards and Weight Classes (ASTM / OIML)
Not all reference weights are equal. The class of a weight tells you how tightly its true mass is controlled — and using the wrong class quietly undermines every calibration that depends on it.
Part of our guide to Mass, Force & Torque Calibration.
What the classes mean
Reference weights are classified by tolerance under standards such as ASTM E617 (Classes 0–7) and OIML R111 (Classes E1, E2, F1, F2, M...). Lower/tighter classes hold smaller tolerances and are used to calibrate higher-accuracy balances or to serve as references for other weights. The class must be tight enough relative to the balance being calibrated to preserve an adequate accuracy ratio.
Choosing and maintaining a set
Match the weight class to the balance’s resolution and your accuracy needs — an analytical balance demands a much tighter class than a shipping scale. Reference weights are themselves calibrated and traceable, handled with care (no bare fingers on precision weights), and recalibrated on an interval, since even stainless weights can change with wear and contamination.
Common questions
Which weight class do I need?
Tight enough that the weight’s tolerance is a small fraction of your balance’s tolerance — analytical balances need high classes (e.g., ASTM Class 1 or OIML E2/F1); coarse scales can use lower classes. We can advise based on your balance.