Perfect Allen wrench selection - GIF

[mklotz]

Generally, I can eyeball one that is obviously too small and one that is obviously too big. Then selecting one that is midway between those two is often correct.

Of course, this assumes the wrenches are ordered in some sort of a holder, not just a handful of wrenches. Neatness in the shop is more than cosmetic; it can save time.

If " those stupid $#@! fractional inch measurements" annoy you enough, label their (presumed sequential) holder with simple progressive integers representing their size in 64ths. (Thus 2 would represent 2/64 = 1/32 and 4 would represent 4/64 = 1/16. Halfway between 2 and 4 would be 3 denoting 3/64.) Deciding the next larger/smaller size would be trivial.

OTOH, if mental arithmetic strikes terror into your heart, convert everything in your shop to metric and never look back. :-)