Width across flats

Width across flats is the distance between two parallel surfaces on the head of a screw, bolt or nut.

The width across flats will define the size of the spanner or wrench needed.

[1] For instance, a bolt for a 1-inch nominal diameter thread might have flats that are 1.5 inches apart.

The width across flats of the fastener (for example screws, nuts, clamps) is nominally the same as that on the tool.

[2] Note that with ISO 272 1982 the width across flats for M10, M12, M14 and M22 were changed from 17, 19, 22 and 32 mm respectively to the current standard.