How do I determine the proper wrench opening or socket size to tighten a specific size of bolt?
Socket sizes and wrench openings for tightening nuts and bolts are determined by the dimension across the flats of either the nut or bolt head. According to the Machinery’s Handbook, 26th edition, “Wrenches are marked with the ‘nominal size of wrench’ which is equal to the basic or maximum width across flats of the corresponding nut”. Although bolts are typically tightened by turning the nut, this would also hold true for bolt heads as well. The wrenches are designed to have enough clearance to fit over the maximum across the flats dimension.
Follow these links for “Across the Flats” dimensions for standard sized nuts and bolts: