Inches to MM Converter
Type inches, as a decimal like 0.375, to get millimeters. The chart below lists the common fractions from 1/64 to 1 inch.
0.5 in =
12.7mm
0.5 inches is 12.7 millimeters.
- 1 in
- 25.4 mm
- 1 mm
- 0.03937 in
How to convert inches to mm
Multiply the inches by 25.4. A fraction has to become a decimal first: 3/8 inch is 3 ÷ 8 = 0.375, and 0.375 × 25.4 = 9.525 mm.
Every sixty-fourth of an inch is 0.396875 mm, so fractional sizes land on tidy, finite decimals: 1/4 inch is 6.35 mm and 5/16 inch is 7.9375 mm. US drill bits, sockets and sheet stock are labelled in these fractions.
Machinists work in thousandths of an inch. One thousandth (0.001 inch) is 0.0254 mm, or 25.4 micrometers, so a tolerance of ±0.005 inch is ±0.127 mm.
millimeters = inches × 25.4
- The inch became exactly 25.4 mm in 1959, when the United States and other English-speaking countries agreed on a common yard of 0.9144 m. Source: NIST, The U.S. survey foot (deprecated from 1 January 2023).
How to estimate inches to mm in your head
Multiplying by 25 is the same as dividing by 4 and shifting the decimal point two places right. For 0.75 inch: 75 ÷ 4 = 18.75 mm, against an exact 19.05. Add about 1 mm for every 2.5 inches to close the gap.
For fractions, remember that 1/16 inch is about 1.6 mm and 1/8 inch about 3.2 mm, then count up: 5/16 is five sixteenths, about 8 mm (7.94 exactly).
Fractional inches to millimeters
Every common fraction from 1/64 to 1 inch, as a decimal inch and in millimeters (all exact).
| Fraction | Decimal inch | Millimeters (mm) |
|---|---|---|
| 1/64 | 0.015625 | 0.396875 |
| 1/32 | 0.03125 | 0.79375 |
| 1/16 | 0.0625 | 1.5875 |
| 3/32 | 0.09375 | 2.38125 |
| 1/8 | 0.125 | 3.175 |
| 5/32 | 0.15625 | 3.96875 |
| 3/16 | 0.1875 | 4.7625 |
| 7/32 | 0.21875 | 5.55625 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.9375 |
Show all 21 rowsShow fewer
| 3/8 | 0.375 | 9.525 |
| 7/16 | 0.4375 | 11.1125 |
| 1/2 | 0.5 | 12.7 |
| 9/16 | 0.5625 | 14.2875 |
| 5/8 | 0.625 | 15.875 |
| 11/16 | 0.6875 | 17.4625 |
| 3/4 | 0.75 | 19.05 |
| 13/16 | 0.8125 | 20.6375 |
| 7/8 | 0.875 | 22.225 |
| 15/16 | 0.9375 | 23.8125 |
| 1 | 1 | 25.4 |
Frequently Asked Questions
What is 1/2 inch in mm?
Half an inch is exactly 12.7 mm (0.5 × 25.4). A 13 mm bolt or socket is 0.3 mm larger.
Which metric drill bit is closest to 1/4 inch?
1/4 inch is 6.35 mm. A 6.5 mm bit is 0.15 mm larger and a 6 mm bit 0.35 mm smaller, so pick 6.5 mm for clearance and 6 mm when the hole must grip.
How do I convert thousandths of an inch to mm?
Multiply by 0.0254. 0.010 inch (ten thou) is 0.254 mm, and 0.125 inch is 3.175 mm.