In Europe the weight classification system adopted is that produced by the International Organization of Legal Metrology (OIML), known as document OIML Recommendation R 111. It describes the metrological aspects of reference standard weights with recommendations for seven classes of weights - from high quality reference standards at Class E1 to those used in industrial settings at Class M3. The full classification includes classes E1, E2, F1, F2, M1, M2, and M3 in tiers of uncertainty or tolerances - for example a Class E1 weight would normally be used as a standard for the calibration of a Class E2 weight (of the same nominal mass).
OIML R 111 describes the principal physical characteristics and metrological requirements for weights which are to be used:
- for the verification of weighing instruments
- for the verification of weights of a lower class of accuracy
- with weighing instruments
The nominal mass of the weights covered by this Recommendation range from 1 milligram (mg) to 50 kilograms (kg) and the document also specifies the methods and equipment to be used for checking the conformity of weights.