Inches to MM Converter

Enter a length in inches to see millimetres. Handy when a US customary dimension must be marked on a metric drawing.

How it works

Multiplying inches by 25.4 converts to millimetres because that factor is the definition of the inch. A half-inch is 12.7 mm; 1/4 inch is 6.35 mm. This is ten times the centimetre result (inches × 2.54), not a separate standard.

Formula

millimetres = inches × 25.4

Worked examples

1 in → 25.4 mm
0.5 in → 12.7 mm

Useful notes

Fractions of an inch (1/8, 1/16) should be entered as decimals (0.125, 0.0625).

FAQ

How many millimetres are in an inch?: Exactly 25.4 millimetres.
How many millimetres is 1/8 inch?: 0.125 × 25.4 = 3.175 mm.
Is this different from inches to centimetres?: Only by a factor of ten. Inches × 2.54 gives centimetres; inches × 25.4 gives millimetres.

Related tools

Inches to CM
CM to Inches

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