A measuring instrument (and its accessories) are characterised by a number known as the class index. This represents the upper limit of the absolute intrinsic error (i.e. due to the instrument alone used under reference conditions) expressed in hundredths of the largest reading that the instrument can give.

Thus, for example, an ammeter of class 0.2 is an instrument for which the absolute intrinsic error does not exceed 0.2% of its largest reading, when used in normal conditions. If this ammeter has 100 divisions, this absolute intrinsic error is thus equal to or less than 0.2/100*100=0.2 of a division.

In the case of an instrument with several ratings, the result remains the same whatever the rating; in contrast the expression of this error in amperes changes with the rating since 0.2 of a division in a scale of 100 divisions represents 0.2/100 of the rated capacity (0.002 A for 1 A rating); (0.01 for 5 A rating).

The values of the class indices are established in the standard NF C 42-100. Instruments with the same class index are said to be of the same precision class. Instruments in class 0.1 or 0.2 are designated as standards; those in class 0.5 are laboratory instruments. Instruments in class 1.5 or 2.5 are control instruments.