Millimeter to Inches Conversion (Mm to Inches) Whenever you need to convert Millimeter to Inches for measuring length, you need to switch from the metric system to the imperial system. This is because a millimeter (mm) is a metric system unit whereas an inch (in) is a unit of length in the imperial system. The formula used to convert Millimeter to Inches: One inch = 25.4 millimetres To convert mm...