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Inches to Millimetres

1 inch = 25.4 millimetres · 1 mm = 0.03937 in

1 inch = 25.4 millimetres

How it’s calculated

mm = in × 25.4

  • Multiply the value in inches by 25.4.
  • To go back, divide by 25.4 (or multiply by 0.03937007874).
  • 1 in = 25.4 mm · 1 mm = 0.03937007874 in
Show worked solutionFactor, unit cancellation and rounding, step by step

  1. Find the conversion factor

    By definition 1 in = 0.0254 m and 1 mm = 0.001 m, so 1 in = 25.4 mm. Source: NIST SP 811 / Handbook 44; BIPM SI Brochure (9th ed.).

  2. Multiply by the factor written as a fraction

    Put in in the denominator so it cancels, leaving mm.

  3. Calculate (unrounded)

  4. Round the answer

    Rounded to 5 significant figures.

Answer: 25.4 mm

1 in in other units

Batch convert in → mm
Results
  1. 1 in = 25.4 mm
  2. 5 in = 127 mm
  3. 10 in = 254 mm
  4. 25 in = 635 mm

Permanent link to this conversion

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Quick examples

  • 1 inch = 25.4 millimetres
  • 5 inches = 127 millimetres
  • 10 inches = 254 millimetres
  • 25 inches = 635 millimetres
  • 100 inches = 2,540 millimetres

About these units

Inch (in)

US & imperial (identical)

Exactly 2.54 cm (international inch, 1959).

1 in = 0.0254 m

Source: NIST SP 811 / Handbook 44

Millimetre (mm)

SI (metric)

Exactly 1/1 000 of a metre.

1 mm = 0.001 m

Source: BIPM SI Brochure (9th ed.)

Inches to millimetres conversion table

inch (in) to millimetre (mm)
Inches (in)Millimetres (mm)
0.1 in2.54 mm
0.5 in12.7 mm
1 in25.4 mm
2 in50.8 mm
3 in76.2 mm
4 in101.6 mm
5 in127 mm
6 in152.4 mm
7 in177.8 mm
8 in203.2 mm
9 in228.6 mm
10 in254 mm
15 in381 mm
20 in508 mm
25 in635 mm
30 in762 mm
40 in1,016 mm
50 in1,270 mm
75 in1,905 mm
100 in2,540 mm
250 in6,350 mm
500 in12,700 mm
1,000 in25,400 mm

Related conversions

Frequently asked questions

How do I convert inches to millimetres?

Use mm = in × 25.4. Multiply the value in inches by 25.4.

How many millimetres are in 1 inch?

1 inch (in) equals 25.4 millimetres (mm).

How many inches are in 1 millimetre?

1 millimetre (mm) equals 0.03937 inches (in); more precisely 0.03937007874.

What is 10 in in mm?

10 inches = 254 millimetres.

What is a inch?

Exactly 2.54 cm (international inch, 1959).

What is a millimetre?

Exactly 1/1 000 of a metre.

Formula: mm = in × 25.4. Factors last reviewed 2026-10-27.