Millimetres to Inches Converter

This converter turns a length in millimetres into its equivalent in inches, using the internationally agreed factor of exactly 0.039370079 inches per millimetre, derived from the exact modern definition of one inch as 25.4 millimetres. Enter a value into the millimetres field and the tool instantly returns the equivalent length in inches, updating as you type, alongside a quick reference showing that 1 mm equals 0.039370079 in and 1 in equals 25.4 mm, so you can work in either direction without switching pages. Below the calculator you will find the formula written out in full, inches equals millimetres multiplied by 0.039370079, or millimetres divided by 25.4 to reverse it, a worked example converting 10 millimetres to 0.393700787 inches, and a quick-reference table covering common values from 1 mm up to 100 mm. Because the conversion is based on the exact definitions of the metre and the international inch, the underlying maths is precise rather than approximate, and the calculator simply rounds the displayed figure for readability. This makes it useful for schoolwork, DIY and trade measurements, imported hardware and fittings sized in inches, engineering drawings, and everyday situations such as checking screen sizes, pipe diameters or timber dimensions where you need a quick, reliable switch between metric and imperial units. For precision engineering or manufacturing tolerances, always confirm the rounding and significant figures your specific job requires.

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0.039370079 in
1 mm =0.039370079 in
1 in =25.4 mm

The formula

Multiply millimetres by 0.039370079 to get inches. In symbols, in = mm × 0.039370079. To go the other way, multiply inches by 25.4 to get millimetres.

Worked example

Convert 10 millimetres to inches: 10 × 0.039370079 = 0.393700787 inches.

Quick reference

MillimetresInches
1 mm0.039370079 in
2 mm0.078740158 in
5 mm0.196850394 in
10 mm0.393700787 in
25 mm0.984251969 in
50 mm1.96850394 in
100 mm3.93700787 in

Frequently asked questions

How many inches are in one millimetre?

One millimetre equals 0.039370079 inches. Multiply the number of millimetres by 0.039370079 to convert.

What is the formula to convert millimetres to inches?

Millimetres times 0.039370079 equals inches. To reverse it, multiply inches by 25.4 to get millimetres.

Is the mm to in conversion exact?

The factor comes from the standard definitions of the metre and the international inch (0.0254 m exactly), so the underlying conversion is exact. Displayed results are rounded for readability.

Who this converter is for

This converter is for students, tradespeople, travellers and anyone who needs to change millimetres into inches quickly and accurately.

What this converter assumes

  • You enter a length in millimetres.
  • The factor used is 0.039370079 inches per millimetre.
  • Results are rounded for display.

Sources

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