The millimeter (mm) is the SI prefix "milli" (10⁻³) applied to the meter: 1,000 mm make 1 m and 10 mm make 1 centimeter. Because the inch is exactly 2.54 cm, it is exactly 25.4 mm.
Conversion
mm = in × 25.4
in = mm ÷ 25.4
mm = cm × 10
Example
A 6 mm bolt diameter is 6 ÷ 25.4 = 0.236 in. A 3/4 inch board is 0.75 × 25.4 = 19.05 mm. Use mm to inches and inches to mm for any value; metric tape-measure markings are explained in how to read a tape measure.
Common mistakes
Dropping the decimal in 25.4 gives errors of nearly 2%. Millimeters and meters differ by a factor of 1,000, so 2,500 mm is 2.5 m, not 25 m.