10.8 DIN ISO 286 ISO-Fundamental tolerances

This ISO Standard represents the basic for a system of nominal dimensions and sizes whereby the table mirrors the calculated values of basic tolerances relating to basic dimensions.
The use of this table is limited to smooth circular cylindrical workpieces or such with two parallel fitting planes or contact areas.
The values attributed to an ISO tolerance grade (IT) specify the tolerance value and hence the tolerance area. With ascending numbers, the size of the tolerance increases.
For identification purpose of the position of the tolerance area in relation to the nominal dimension (zero), the number chosen as tolerance grade IT is preceeded by a letter.
Tolerance area H is the most common value for bores. It specifies that the minimum dimension of the bore corresponds to the nominal dimension.
The permissible maximum dimension corresponds to the nominal dimension plus the IT tolerance.

Examples:
bore 20 H7 = 20 +0.021/0 | bore 8 H11 = 8 +0.090/0
min. dimension: 20.000 | min. dimension: 8.000
max. dimension: 20.021 | max. dimension: 8.090