How to Convert MM to Inches

One inch equals exactly 25.4 millimeters. That single relationship drives every conversion between the two units.

The formula

inches = mm ÷ 25.4
mm = inches × 25.4

Worked example

Convert 100 mm to inches: 100 ÷ 25.4 = 3.937". As a fraction, that's approximately 3 15/16".

Fractions of an inch

Imperial measurements often use fractions like 1/16", 1/8", 1/4". To convert a decimal inch to the nearest 1/64", multiply the decimal by 64, round to the nearest whole number, then reduce the fraction.

Common mistakes

Using 25 instead of 25.4 — introduces a ~1.6% error.
Confusing millimeters (mm) with centimeters (cm). 1 cm = 10 mm.
Rounding too early. Keep 3–4 decimal places until the final step.