The exact factor, and a shortcut you can do in your head
One inch is exactly 25.4 mm, fixed by the 1959 international yard and pound agreement, so this conversion carries no rounding at all. For a rough figure, divide the millimetres by 25, or remember that 100 mm is close to 4 inches (3.94). Going the other way, 1 mm is about 0.04 inch.
Common sizes in inches
| mm | Inches (decimal) | Nearest fraction |
|---|---|---|
| 1 mm | 0.039" | ~1/25" |
| 3 mm | 0.118" | ~1/8" |
| 6.35 mm | 0.250" | 1/4" |
| 10 mm | 0.394" | ~25/64" |
| 12.7 mm | 0.500" | 1/2" |
| 19.05 mm | 0.750" | 3/4" |
| 25.4 mm | 1.000" | 1" |
| 50.8 mm | 2.000" | 2" |
| 100 mm | 3.937" | ~3 15/16" |
The clean fractions sit on odd millimetre values: 1/4" is 6.35 mm, 1/2" is 12.7 mm and 3/4" is 19.05 mm — the common bolt and pipe sizes.
Inch fractions, converted
US measurements use halving fractions rather than decimals. These are the sizes stamped on sockets, drill bits and rules.
| Fraction | Decimal | mm |
|---|---|---|
| 1/16" | 0.0625" | 1.588 mm |
| 1/8" | 0.125" | 3.175 mm |
| 1/4" | 0.250" | 6.350 mm |
| 3/8" | 0.375" | 9.525 mm |
| 1/2" | 0.500" | 12.700 mm |
| 3/4" | 0.750" | 19.050 mm |
A common slip: 3/8" is not 0.38". Three eighths is 0.375", which is 9.525 mm.
The mistake to avoid
- M6 is not 1/4". A 6 mm metric bolt (M6) and a 1/4" bolt (6.35 mm) are close in diameter but the threads differ, so they will not screw into each other's nuts.
- Don't confuse mm and cm. 10 mm is 1 cm, so mixing them up throws the answer off by a factor of ten.
Common questions
How many millimetres are in an inch?
Exactly 25.4 mm. This is a defined value from the 1959 international agreement, not a rounded approximation.
How do I convert mm to inches?
Divide by 25.4. For example, 50 mm divided by 25.4 is 1.97 inches, just under 2 inches.
What is 10 mm in inches?
10 mm is 0.394 inches, a shade under 25/64 of an inch. A handy anchor is that 100 mm is 3.94 inches, close to 4.
