I have often been confused and irritated by the key sizes for nuts and bolts and the standard sizes. I used to work with these all day long 20 years ago, but now somehow they never seem right.... Finally I looked up the official tables and yes, they have changed. The international standard
did change for some sizes, about 20 years ago. This was done because prior to that, different standards where used in different countries. Also, it was an attempt to make key sizes the same for metric an US sizes.
Leaving out the tiny ones and really big ones we get this table:
| Preferred standard nut/bolt sizes | Standard key size | Obsolete key size, > 20 years old |
| M3 | 5.5mm | |
| M4 | 7mm | |
| M5 | 8mm | |
| M6 | 10mm | |
| M8 | 13mm | 14mm |
| M10 | 16mm | 15mm,17mm |
| M12 | 18mm | 19mm |
| M16 | 24mm | |
So in reality there are not that many sizes we need in the toolbox. Eight different will be enough in 99% of all cases, unless we are into electronics or big constructions or some oddball non-standard sizes.
When will we see hex key kits with just the standard sizes? I'd rather have a kit with two of each of the standard key sizes than all those that are never used. How often did you use 9mm, 11mm, 12mm found in many key sets?
