Calcylator
Inches to Millimetres

Inches to millimetres:
multiply by 25.4

Convert inches to millimetres for pipes, screws, screens and drawings, with a chart of common fractions and the mistakes to avoid.

Calcylator Editorial Team

Updated · 5 min read

How many millimetres are in an inch?

One inch is exactly 25.4 millimetres. This is not a rounded figure: since 1959 the inch has been defined as 25.4 mm by agreement between the English-speaking countries, so every conversion between the two units uses the same fixed number.

That makes inches to mm one of the easiest length conversions. There is no long decimal to remember, only a single multiplication. A millimetre is a thousandth of a metre, so 25.4 mm is also 2.54 cm, the figure you may know from converting inches to centimetres.

The inches to mm formula

Inches to millimetres =mm = inches × 25.4
mm:
length in millimetres
inches:
length in inches
25.4:
millimetres in one inch (exact)
To go the other way, divide: inches = mm ÷ 25.4.

  • Length

    5.5 inches

  • Multiply

    5.5 × 25.4

Length in mm

139.7 mm

5 × 25.4 = 127, and 0.5 × 25.4 = 12.7, so 127 + 12.7 = 139.7.

Going back is just as simple. A 50 mm part is 50 ÷ 25.4 = 1.9685 inches, which rounds to 1.97 inches. For a quick mental check, remember that 25 mm is almost exactly one inch, so any answer should be close to the mm value divided by 25.

  1. Write the length in inches as a decimal (turn 1 1/4 into 1.25).
  2. Multiply by 25.4.
  3. Round to the precision the job needs: usually one decimal for drawings and two or three for machining.
  4. Check the answer against your rough estimate of inches × 25.

Chart: common inch fractions in millimetres

Tools, pipes and fasteners are often sized in fractions of an inch. These values are exact or rounded to three decimals.

Inch fractions to millimetres
InchesDecimalMillimetres
1/80.1253.175
1/40.256.35
3/80.3759.525
1/20.512.7
5/80.62515.875
3/40.7519.05
7/80.87522.225
1125.4
1 1/41.2531.75
1 1/21.538.1

For whole inches the pattern is easy to extend: 2 inches is 50.8 mm, 3 inches is 76.2 mm, 4 inches is 101.6 mm, 6 inches is 152.4 mm and 12 inches (one foot) is 304.8 mm.

A decimal example: 0.571 inches

Measurements are not always tidy fractions. A part listed as 0.571 inches converts the same way: 0.571 × 25.4 = 14.5034 mm, or about 14.5 mm. Keep the extra digits while you calculate and round only at the end, otherwise small errors add up across several parts.

Common mistakes when converting inches to mm

  • Dividing by 25.4 when you should multiply. A length in inches always has a larger number in millimetres, so if your answer is smaller, you divided.
  • Treating 1 1/4 as 1.4. A quarter is 0.25, so 1 1/4 inches is 1.25 inches or 31.75 mm.
  • Using 25 instead of 25.4. The shortcut is fine for estimates but gives a 1.6 percent error, which is about 4 mm on a 10-inch length.
  • Mixing up millimetres and centimetres. 25.4 mm is 2.54 cm, so shift the decimal point one place.

Common questions

How many mm is 1 inch?

One inch is exactly 25.4 millimetres. This is an exact definition rather than a rounded value, so you can use 25.4 in any calculation without worrying about loss of accuracy. It is also equal to 2.54 centimetres.

How do I convert inches to mm by hand?

Multiply the inches by 25.4. For example, 3 inches is 3 × 25.4 = 76.2 mm. For a quick estimate, multiply by 25 and add a little, since 25.4 is slightly more than 25. Convert fractions to decimals first.

How many mm is 1 1/4 inch?

One and a quarter inches is 1.25 inches, and 1.25 × 25.4 = 31.75 mm. Add the whole part, 25.4 mm, and the quarter, 6.35 mm, to check the result: 25.4 + 6.35 = 31.75 mm.

How do I convert mm to inches?

Divide the millimetres by 25.4. A 100 mm length is 100 ÷ 25.4 = 3.937 inches. As a quick check, every 25.4 mm adds one inch, so 100 mm should be a little under 4 inches.

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