In order to convert centimeters to inches, you use the following equation:
1 inches = 2.54 cm
If you have to convert, for example, 4 inches, you proceed as follow:
4 inches* (2.54cm/1 inches) = 10.16 cm
That is, you multiply the quantity that you want to convert, by the quotient between 2.54cm and 1 inches.
