Rough rule of thumb is that the head size will be one and a half times the size of the thread. Where the "wheels come off" is that there are European, Japanese and American versions of metric...

6mm thread (about 1/4" shank) - almost universal - 10mm wrench

8mm thread (like 5/16") - anything Euro or Japanese = 12mm, American will be 13mm so a 1/2" wrench can be used...

10mm thread (heavy 3/8") - mostly a 15mm wrench. Some American manufacturers will make it 16mm to accomodate a 5/8" wrench - GRRRR. Most wrench sets (esp made anywhere else in the world) don't have a 16mm because they aren't retarded enough to worry about a 5/8" "spanner"...

12mm thread (1/2") - actually - almost universally a 19mm wrench. The 'convention" in metric is that once they get to 15mm, they go up in 2's (17, 19, 21, then 3's - 24, 27, 30). So most hardware is almost always a 19mm head on a 12mm bolt, but Deere in their infinite wisdom decided to spec some stuff with an 18mm head - GRRRR.

16mm thread (5/8") 24mm wrench - 15/16" works

20mm thread (fat 3/4" - 30 mm wrench - 1and 3/16" socket out of the 3/4" drive set...


Thats a start - with some editorial.

Luke