Divide millimetres by 25.4 to get inches. Drill bits, wrench sizes, rainfall totals and jewellery are often given in millimetres.
How to use the mm to Inches Converter
- Type the number of millimetres into the Millimetres field. Decimals such as 12.7 are fine.
- Read the answer in Inches – it updates as you type, with no button to press.
- To convert the other way, type into Inches and the millimetres are recalculated.
- Press Copy results to copy both values with the page title, or Copy link to share this exact conversion.
This page converts only mm ⇄ in. Need a different unit? Use the general length converter, where you can choose any “from” and “to” unit.
How it works
Both units measure length, so they are always in the same ratio: one millimetre equals 0.039370079 inches. The calculator multiplies the number of millimetres you type by 0.039370079 to give inches, and when you type into the inches field it divides by the same factor to give millimetres. Because the ratio never changes, doubling the millimetres always doubles the inches. Results are calculated at full precision and rounded only for display (up to eight decimal places, with very large or small values shown in scientific notation).
How to convert millimetres to inches
- Write down the value in millimetres, for example 25.4 mm.
- Multiply by the conversion factor 0.039370079 (because 1 millimetre = 0.039370079 in).
- 25.4 × 0.039370079 = 1 in.
- To go back from inches to millimetres, divide by 0.039370079 – or multiply by 25.4.
mm to in conversion table
| Millimetres | Inches | Inches | Millimetres |
|---|---|---|---|
| 1 mm | 0.039370079 in | 1 in | 25.4 mm |
| 2 mm | 0.078740157 in | 2 in | 50.8 mm |
| 5 mm | 0.19685039 in | 5 in | 127 mm |
| 6 mm | 0.23622047 in | 10 in | 254 mm |
| 8 mm | 0.31496063 in | 20 in | 508 mm |
| 10 mm | 0.39370079 in | 50 in | 1,270 mm |
| 15 mm | 0.59055118 in | 100 in | 2,540 mm |
| 20 mm | 0.78740157 in | 1,000 in | 25,400 mm |
| 25 mm | 0.98425197 in | ||
| 25.4 mm | 1 in | ||
| 50 mm | 1.9685039 in | ||
| 100 mm | 3.9370079 in |
What is a millimetre?
A millimetre is one thousandth of a metre. Engineering drawings, construction plans, rainfall, screws, bolts and jewellery are all dimensioned in millimetres.
What is an inch?
The inch has been defined as exactly 25.4 mm since 1959. It is used in the U.S. for almost everything and worldwide for screen sizes, wheel and tyre sizes, pipe threads and many tools.
Quick mental conversion
For a fast estimate, multiply millimetres by about 0.039 to get inches; the error is about 0.9%. Use the calculator above when you need the exact figure.
Common mistakes to avoid
Mixing the international foot (0.3048 m) with the old u.s. survey foot, confusing nautical and statute miles, and writing decimal feet such as 5.5 ft as “5 ft 5 in” (it is 5 ft 6 in).
Formula
1 mm = 0.039370079 in1 in = 25.4 mmin = mm × 0.039370079Worked example
Inputs — Millimetres: 25.4 mm.
Results — Inches = 1 in.
Frequently asked questions
How many inches is 10 mm?
About 0.394 in.
How many mm in an inch?
Exactly 25.4.
How do I convert millimetres to inches?
Multiply the number of millimetres by 0.039370079. For example, 25.4 mm × 0.039370079 = 1 in.
How do I convert inches back to millimetres?
Divide by 0.039370079 (or multiply by 25.4), or simply type the value into the inches field above.
What is 10 mm in inches?
10 mm = 0.39370079 in.
What is 100 mm in inches?
100 mm = 3.9370079 in.
Is this conversion exact?
Yes. The factor comes from the official definitions of both units (NIST and the SI), so the only difference you may see is rounding in the last displayed digit.
Can I convert to other units on this page?
This page converts only between millimetres and inches. To pick any length unit, use the general length converter linked under Related calculators.
What formula does the mm to Inches Converter use?
1 mm = 0.039370079 in.