Since sometime in the early 1900s, the standard US sizes for hexagonal bolt heads and nuts have come in two flavors, SAE and Heavy. The SAE heads are 1 1/2 times the bolt shank diameter, the Heavy heads 1/8 inch larger.
A common example, for a bolt with a 3/8 inch diameter shank: An SAE standard head would be 1 1/2 x 3/8 inch = 9/16 inch. A Heavy standard head would be 9/16 inch + 1/8 inch.
Under both of these formulae, a bolt with a "odd sixteenths" shank diameter would have a "odd thirty-seconds" head.
For a bolt with a 5/16 inch shank, an SAE head would be 1 1/2 x 5/16 inch = 15/32 inch, and a Heavy head would be 15/32 + 1/8 inch = 19/32 inch.
For a bolt with a 7/16 inch shank, an SAE head would be 1 1/2 x 7/16 inch = 21/32 inch, and a Heavy head would be 21/32 inch + 1/8 inch = 25/32 inch.