A pound is a unit of mass. A millimetre is a unit of distance. The two units are therefore incompatible.
31 mm = 3.1 cm
1 cm = 10 mm Therefore 3.1 cm = 31 mm
1000 mm equals one meter
6.3 centimeters equals 63 mm.
31 inches is 787.4 millimeters.
15 mm
457.2 mm.
63 mm
6,800 mm
84 mm ;-)
1 km equals 100,000 mm
47 mm equals 4.7 cm