Inches to MM Converter
Convert inches to millimetres — 25.4 mm to the inch, the workshop conversion. Results update as you type; exact factors shown.
One inch is 25.4 millimetres — see the exact factor, not a rounded rule of thumb.
How the inches to mm 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 |
|---|---|---|
| 0.125 in | 3.175 mm | inchs to millimetres |
| 0.25 in | 6.35 mm | inchs to millimetres |
| 0.5 in | 12.7 mm | inchs to millimetres |
| 1 in | 25.4 mm | inch to millimetres |
| 1.5 in | 38.1 mm | inchs to millimetres |
| 2 in | 50.8 mm | inchs to millimetres |
| 3 in | 76.2 mm | inchs to millimetres |
| 4 in | 101.6 mm | inchs to millimetres |
| 6 in | 152.4 mm | inchs to millimetres |
| 8 in | 203.2 mm | inchs to millimetres |
| 12 in | 304.8 mm | inchs to millimetres |
| 24 in | 609.6 mm | inchs to millimetres |
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 mm in an inch?
Exactly 25.4.
What is half an inch in mm?
12.7 mm.
What is ⅛ inch?
3.175 mm.
Why do machinists prefer mm?
Because whole millimetres avoid the fractions — 3.175 mm is easier to hold than a sixteenth and a bit.
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.