MM to Inches Converter
Convert millimeters to inches — 25.4 mm to the inch, the everyday unit of workshops and drawings. Results update as you type; exact factors shown.
One millimeter 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 meter, 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 US uses customary units (US gallon, short ton, °F) in daily life; NIST SP 811 gives the exact SI factors.
Common length conversions
| From | To | Units |
|---|---|---|
| 1 mm | 0.0394 in | millimeter to inchs |
| 5 mm | 0.1969 in | millimeters to inchs |
| 10 mm | 0.3937 in | millimeters to inchs |
| 12 mm | 0.4724 in | millimeters to inchs |
| 16 mm | 0.6299 in | millimeters to inchs |
| 20 mm | 0.7874 in | millimeters to inchs |
| 25 mm | 0.9843 in | millimeters to inchs |
| 25.4 mm | 1 in | millimeters to inchs |
| 50 mm | 1.9685 in | millimeters to inchs |
| 100 mm | 3.937 in | millimeters to inchs |
| 150 mm | 5.9055 in | millimeters to inchs |
| 300 mm | 11.811 in | millimeters to inchs |
Units in this converter and their exact factors
| Unit | Symbol | One unit in meters |
|---|---|---|
| millimeter | mm | 0.001 meter |
| centimeter | cm | 0.01 meter |
| meter | m | 1 meter |
| kilometer | km | 1,000 meter |
| inch | in | 0.0254 meter |
| foot | ft | 0.3048 meter |
| yard | yd | 0.9144 meter |
| mile | mi | 1,609.344 meter |
| nautical mile | nmi | 1,852 meter |
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 millimeters?
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 Institute of Standards and Technology (NIST) — United States's national measurement authority.
Last checked: September 2026. Conversion factors are fixed by definition and do not change with tax years or regulations.