General Tolerances: Straightness and Flatness

Straightness and flatness are included in the general tolerances of ISO 2768-2:1989. They define how much an edge or surface may deviate from its ideal form when no separate geometric or dimensional tolerance is specified.

• Straightness refers to the deviation of an individual edge or axis from a straight line. The general tolerance ensures that, for example, an axis or machined edge remains sufficiently straight for practical requirements.
• Flatness describes how much an entire surface may deviate from an ideal plane. This is important, for example, on mating surfaces, where a slight unevenness can affect how the parts fit together.

Drawings do not include separate symbols for straightness and flatness when general tolerances are applied. Instead, a note such as ISO 2768-mK is sufficient, in which case all corresponding features automatically fall within the deviation limits specified by the standard.

General Tolerance Table: Straightness and Flatness

Tolerance class	Straightness and flatness tolerances for nominal length ranges
Designation	>		10	30	100	300	1000
	≤	10	30	100	300	1000	3000
H		0.02	0.05	0.1	0.2	0.3	0.4
K		0.05	0.1	0.2	0.4	0.6	0.8
L		0.1	0.2	0.4	0.8	1.2	1.6

The values in the table are given in millimeters (mm).