A millimeter is one one-thousandth of a meter. Ten millimeters equals one centimeter. One thousand millimeters equals one meter. It is roughly the thickness of a paperclip. It is the smallest measurement on a normal ruler. One inch is 25.4 millimeters.
There are 25.4 centimeters in one inch. Therefore to get amount of millimeters in inches, value in inches has to be multiplied by amount of millimeters in one inch: 1 inch = [inches] * 25.4 = 1 * 25.4 = 25.4 millimeters