Imperial to metric conversion:

Inches   Millimetres
1 1/2" = 38mm
1 3/4" = 48mm
2" = 53mm
2 1/4" = 58mm
2 1/2" = 63mm
2 3/4" = 68mm
3" = 78mm
3 1/2" = 88mm
4"
= 103mm
5" = 128mm
6" = 153mm
7" = 177mm
9" = 228mm
10" = 254mm
12" = 305mm
15" = 381mm
16" = 406mm
18" = 457mm