Spanner sizes
Information on the across flats dimensions of nuts and bolts in common use seems quite hard to find. There are so many thread types still in circulation, and almost without exception, all use quite independent across flats sizes. Metric threads use whole metric numbers. 17, 19, 20 etc. UNF/UNC go for Imperial fractional sizes. 5/8", 1/2" etc. BA, Whit & BSF seem to use a decimal size with no easy fractional base. 0.445", 0.710", 1.100" etc. The now obsolete across flats sizes relate direct to the spanner !! Comparison of across flats dimensions shows that, for correct practice, there are very few spanners that are truly multi thread. As with all matters engineering the empirical method is usually the best. If the spanner/socket is a good fit, use it. No official across flats dimensions are given for ME ( Model Engineer ) threads as nuts and bolts are not commercially available. Figures given are the editors own based on the BA formulae. In the BA sizes it is possible to buy nuts etc with a size less across flats. ie: 2BA with 3BA dimensions. This gives a better scale effect, where strength is not important. The Across Flats zipped file gives Spanner ( & Socket ) sizes in ascending order, very good to see if your "nearest size" is near enough. If you are considering the manufacture of odd nuts & bolts, make sure that a suitable hexagon bar is available. Milling flats on dozens of nuts & bolts is not my idea of a good time ? Go to Across Flats Database.
Note:- At one time Whit & BSF across flats sizes were the same. ie a 5/16"Whit & 5/16 BSF across flats was the same. As an economy measure during WWII it was decided to reduce the across flats sizes of Whitworth nute and bolts by one size. Thus a 5/16" Whit across flats was the same as a 3/8" BSF. Also note that the core diameter for a Whitworth thread is less than the same size in BSF. (Coarser thread pitch, hence deeper threads)