Verified by Rachel Mayfield, Supply Chain Analyst - June 2026
Photo by Daniel Smyth on Pexels
Fraction-to-millimetre conversions for imperial drill bits and bolt sizes from 1/64 inch to 1 inch, with the decimal-inch equivalent. Fraction to MM Conversion Chart (Drill and Bolt Sizes) One inch is exactly 25.4 mm, so a 1/2 inch drill is 12.7 mm and a 1/4 inch drill is 6.35 mm. The chart bridges imperial-marked tooling and metric drawings so a fractional drill can be matched to a metric hole size and the reverse.
| Fraction (inch) |
Decimal (inch) |
Millimetres (mm) |
| 1/64 |
0.0156 |
0.397 |
| 1/32 |
0.0312 |
0.794 |
| 3/64 |
0.0469 |
1.191 |
| 1/16 |
0.0625 |
1.587 |
| 5/64 |
0.0781 |
1.984 |
| 3/32 |
0.0938 |
2.381 |
| 1/8 |
0.1250 |
3.175 |
| 5/32 |
0.1562 |
3.969 |
| 3/16 |
0.1875 |
4.762 |
| 7/32 |
0.2188 |
5.556 |
| 1/4 |
0.2500 |
6.350 |
| 9/32 |
0.2812 |
7.144 |
| 5/16 |
0.3125 |
7.938 |
| 3/8 |
0.3750 |
9.525 |
| 7/16 |
0.4375 |
11.112 |
| 1/2 |
0.5000 |
12.700 |
| 9/16 |
0.5625 |
14.287 |
| 5/8 |
0.6250 |
15.875 |
| 11/16 |
0.6875 |
17.462 |
| 3/4 |
0.7500 |
19.050 |
| 7/8 |
0.8750 |
22.225 |
| 1 |
1.0000 |
25.400 |
How to convert fractions to millimetres
Divide the fraction to get the decimal inch, then multiply by 25.4 to get millimetres. A 3/8 inch drill is 0.375 inch, and 0.375 times 25.4 is 9.525 mm. To go the other way, divide the millimetre figure by 25.4 for the decimal inch, then find the nearest fraction. There's rarely an exact fractional match for a round metric size, so pick the closest drill and check whether the small difference matters for the fit.
The closest common swaps are worth memorising: 6 mm sits between 15/64 (5.953 mm) and 1/4 (6.35 mm); 10 mm is just under 25/64 (9.922 mm); and 1/2 inch (12.7 mm) has no exact metric drill, with 12.5 mm and 13 mm the nearest stock sizes. For tapping, always use the metric tap drill chart rather than rounding a fractional drill.
Where this data comes from
- Conversions are exact: 1 inch = 25.4 mm by international definition (1959)
- BS 328 - Twist drills and combined drills (fractional and metric drill series)
Frequently Asked Questions
What is 1/2 inch in mm?
1/2 inch is exactly 12.7 mm. The conversion is the decimal value (0.5 inch) multiplied by 25.4, since one inch is defined as 25.4 mm. There is no exact metric drill at 12.7 mm, so 12.5 mm or 13 mm are the nearest stock sizes.
How do you convert inches to mm?
Multiply the inch value by 25.4 to get millimetres, because one inch is defined as exactly 25.4 mm. For a fraction, divide it first to get the decimal: 3/8 inch is 0.375, and 0.375 times 25.4 equals 9.525 mm. To convert mm to inches, divide by 25.4.
What metric drill is closest to 1/4 inch?
A 1/4 inch drill is 6.35 mm. The nearest metric stock drills are 6 mm (0.236 inch, slightly smaller) and 6.5 mm (0.256 inch, slightly larger). For a clearance hole, 6.5 mm is usually the safer choice; for a tight fit, 6 mm is closer.