MM to Inches Converter
Convert millimetres to inches — 25.4 mm to the inch, the everyday unit of workshops and drawings. Results update as you type; exact factors shown.
One millimetre is 0.0394 inchs — see the exact factor, not a rounded rule of thumb.
How the mm to inches converter works
Every length is converted through the metre, the SI base unit of length. The inch has been defined as exactly 25.4 mm since the 1959 international yard and pound agreement, so a foot is exactly 0.3048 m, a yard 0.9144 m and a statute mile 1,609.344 m. The nautical mile is 1,852 m by international definition.
The UK is metric for most trade and science but keeps miles, pints, stone and mpg (with the imperial gallon) in daily life.
Common length conversions
| From | To | Units |
|---|---|---|
| 1 mm | 0.0394 in | millimetre to inchs |
| 5 mm | 0.1969 in | millimetres to inchs |
| 10 mm | 0.3937 in | millimetres to inchs |
| 12 mm | 0.4724 in | millimetres to inchs |
| 16 mm | 0.6299 in | millimetres to inchs |
| 20 mm | 0.7874 in | millimetres to inchs |
| 25 mm | 0.9843 in | millimetres to inchs |
| 25.4 mm | 1 in | millimetres to inchs |
| 50 mm | 1.9685 in | millimetres to inchs |
| 100 mm | 3.937 in | millimetres to inchs |
| 150 mm | 5.9055 in | millimetres to inchs |
| 300 mm | 11.811 in | millimetres to inchs |
Units in this converter and their exact factors
| Unit | Symbol | One unit in metres |
|---|---|---|
| millimetre | mm | 0.001 metre |
| centimetre | cm | 0.01 metre |
| metre | m | 1 metre |
| kilometre | km | 1,000 metre |
| inch | in | 0.0254 metre |
| foot | ft | 0.3048 metre |
| yard | yd | 0.9144 metre |
| mile | mi | 1,609.344 metre |
| nautical mile | nmi | 1,852 metre |
FAQFrequently asked questions about length conversion
How many inches is 1 mm?
0.03937 inches, about one twenty-fifth.
What is 25.4 mm in inches?
Exactly one inch.
How thick is 12 mm in inches?
0.472 inches, just under half an inch.
Why do drawings use millimetres?
Because they avoid decimals for most part sizes — 12.5 mm is easier to read and machine than 0.492 inches.
What is the nearest inch fraction to 10 mm?
Three-eighths of an inch is 9.525 mm, so 10 mm is a little larger.
Where these figures come from
- BIPM — The International System of Units (SI Brochure, 9th edition) — definitions of the base and derived units.
- NIST Special Publication 811 — Guide for the Use of the International System of Units — the exact conversion factors used here.
- National Physical Laboratory (NPL) — United Kingdom's national measurement authority.
Last checked: September 2026. Conversion factors are fixed by definition and do not change with tax years or regulations.