What formulas convert between inches and millimeters?

What formulas convert between inches and millimeters?

Answer: MM = inches * 25.4. INCHES = mm * .0393.
Convert from inches to millimeters:  Example of 10″ board: 10″ * 25.4 = 254mm
Convert from millimeters to inches:  Example of 254mm board: 254mm * .0393 = ~10″