First, you need to go from inches to centimeters, since the conversion factor is widely known. There are 2.54 centimeters in 1 inch. Now, millimeters can be calculated. There are 10 millimeters per centimeter. Multiply 2.54 centimeters by 10 millimeters, and you will have the number of millimeters per inch which calculates to be 254 millimeters.