For every bolt or screw diameter, there are infinite possibilities for the inner diameter, outer diameter and thickness of the accompanying flat washer. In order to make the selection of a flat washer easier, certain standards have evolved.
Commercial Flat Washers are not dimensionally equivalent to USS (United States Standard) pattern, however, some sizes do match the SAE (Society of Automotive Engineers) pattern. The inner diameter is large enough to accommodate a bolt shank at the high side of tolerance. In the interests of economy when flat washers are made from stainless or nickel-based alloys, outer diameters and thicknesses are only as large as functionally needed.
Therefore, there is no governing spec for commercial pattern washers. Dimensions will be dependent on each supplier’s standards.