The millimeter is a unit of length in the metric system equal to one thousandth of a meter. 1 millimeter is about the thickness of a paper clip. It is roughly one-sixteenth of an inch. It is the smallest measurement on a normal ruler. Ten millimeters equals one centimeter. One thousand millimeters equals a meter.
¼ Inch = 6.35mm
½ inch = 12.70mm
1 inch = 25.40mm
Please refer to http://www.metric-conversions.org/length/millimeters-to-inches.htm to convert millimeters to inches.