Standard (Inches) to metric (mm) conversion table

Very good info. I have & still do use 1/2" for 13mm, 3/4" for 19mm & 13/16" for 21mm for some fasteners & vice versa, they are a good fit.
Don't forget. 8mm for 5/16" ,14mm for 9/16", 16mm for 5/8"