Metric flat washers are used under bolt heads or nuts to increase the bearing area or offer greater hardness, so that the nut or bolt can be rotated without distortion of the bearing surface. Flat washers are also used to give tightness to a joint, to prevent leakage, and especially to distribute pressure under the head of a nut or bolt.

Fender flat washers have a large outside diameter in comparison with the inside diameter (hollow section).