Quote Originally Posted by EddyCurrent View Post
Yes but I meant in a proper file header, not the code itself, so that Mach3 for example would scale it automatically.
Ok last word from me on this as it's not the OP's problem.!!

Mach won't scale it automaticly because it's native units are set to metric so 1 in G-code equals 1 unit and if thats metric it's 1mm or 1" if imperial.
The G20/21 doesn't change any of mach's settings so the native units are the same. It's mostly to identify the units not set or change them.