The OIML weight classes are defined as follows:

 

Class E1:- Weights intended to ensure traceability between national mass standards (with values derived from the International Prototype of the kilogram) and weights of class E2 and lower. Class E1 weights or weight sets shall be accompanied by a calibration certificate.

 

Class E2:- Weights intended for use in the verification or calibration of class F1 weights and for use with weighing instruments of special accuracy class I. Class E2 weights or weight sets shall be accompanied by a calibration certificate. They may be used as class E1 weights if they comply with the requirements for surface roughness, magnetic susceptibility and magnetization for class E1 weights, and if their calibration certificate gives the appropriate data.

 

Class F1:- Weights intended for use in the verification or calibration of class F2 weights and for use with weighing instruments of special accuracy class I and high accuracy class II.

 

Class F2:- Weights intended for use in the verification or calibration of class M1 and possibly class M2 weights. They also intended for use in important commercial transactions (e.g. precious metals and stones) on weighing instruments of high accuracy class II.

 

Class M1:- Weights intended for use in the verification or calibration of class M2 weights, and for use with weighing instruments of medium accuracy class III.

 

Class M2:- Weights intended for use in the verification or calibration of class M3 weights and for use in general commercial transactions and with weighing instruments of medium accuracy class III.

 

Class M3:- Weights intended for use with weighing instruments of medium accuracy class III and ordinary accuracy class IIII.

 

Classes M1–2 and M2–3:- Weights from 50 kg to 5 000 kg of lower accuracy intended for use with weighing instruments of medium accuracy class III.