How to use the mm to inches converter
Type a size in millimeters. The big number is the exact decimal inch value; below it are the closest fractions of an inch at three levels of precision, each with the difference from the true size in millimeters, so you can see whether "close enough" really is. For the reverse conversion choose Inches → millimeters and type decimal inches: 1/2 in is 0.5, 3/8 in is 0.375, 5/16 in is 0.3125.
The formula
An inch is exactly 25.4 millimeters, so inches = millimeters ÷ 25.4 and millimeters = inches × 25.4. There is no rounding in the definition: 25.4 is exact, which is why 1/64 in = 0.396875 mm, with no more decimals hiding behind it.
Why fractions of an inch, and why 1/64
Inch rulers and tape measures divide the inch in halves: 1/2, 1/4, 1/8, 1/16 and, on finer rules, 1/32 and 1/64. A sixty-fourth of an inch is about 0.4 mm — about as fine a division as is practical to read on a rule — so 1/64 is the usual limit for fractional drill bits and woodworking. Beyond that, machinists switch to decimal inches ("thou", 0.001 in = 0.0254 mm). The converter rounds to the nearest mark at each level and then reduces the fraction, so 32/64 shows as 1/2.
Worked example: 10 mm
- 10 ÷ 25.4 = 0.3937 in.
- Nearest 1/16: 0.3937 × 16 = 6.3 → 6/16 = 3/8 in (9.525 mm, 0.48 mm too small).
- Nearest 1/32: 0.3937 × 32 = 12.6 → 13/32 in (10.32 mm, 0.32 mm too large).
- Nearest 1/64: 0.3937 × 64 = 25.2 → 25/64 in (9.92 mm, only 0.08 mm too small).
So a 25/64 in drill bit makes a hole that a 10 mm bolt will just fit through, while a 3/8 in bit will not.
Millimeters to inches, 1 to 25 mm
| mm | Inches | Nearest 1/64 in | Fraction − true (mm) |
|---|---|---|---|
| 1 | 0.0394 | 3/64 | +0.19 |
| 2 | 0.0787 | 5/64 | −0.02 |
| 3 | 0.1181 | 1/8 | +0.17 |
| 4 | 0.1575 | 5/32 | −0.03 |
| 5 | 0.1969 | 13/64 | +0.16 |
| 6 | 0.2362 | 15/64 | −0.05 |
| 7 | 0.2756 | 9/32 | +0.14 |
| 8 | 0.3150 | 5/16 | −0.06 |
| 9 | 0.3543 | 23/64 | +0.13 |
| 10 | 0.3937 | 25/64 | −0.08 |
| 11 | 0.4331 | 7/16 | +0.11 |
| 12 | 0.4724 | 15/32 | −0.09 |
| 13 | 0.5118 | 33/64 | +0.10 |
| 14 | 0.5512 | 35/64 | −0.11 |
| 15 | 0.5906 | 19/32 | +0.08 |
| 16 | 0.6299 | 5/8 | −0.12 |
| 17 | 0.6693 | 43/64 | +0.07 |
| 18 | 0.7087 | 45/64 | −0.14 |
| 19 | 0.7480 | 3/4 | +0.05 |
| 20 | 0.7874 | 25/32 | −0.16 |
| 21 | 0.8268 | 53/64 | +0.03 |
| 22 | 0.8661 | 55/64 | −0.17 |
| 23 | 0.9055 | 29/32 | +0.02 |
| 24 | 0.9449 | 15/16 | −0.19 |
| 25 | 0.9843 | 63/64 | +0.00 |
Common fractional inch sizes in millimeters
| Fraction | Decimal inches | mm |
|---|---|---|
| 1/16 in | 0.0625 | 1.59 |
| 1/8 in | 0.1250 | 3.18 |
| 3/16 in | 0.1875 | 4.76 |
| 1/4 in | 0.2500 | 6.35 |
| 5/16 in | 0.3125 | 7.94 |
| 3/8 in | 0.3750 | 9.52 |
| 7/16 in | 0.4375 | 11.11 |
| 1/2 in | 0.5000 | 12.7 |
| 9/16 in | 0.5625 | 14.29 |
| 5/8 in | 0.6250 | 15.88 |
| 3/4 in | 0.7500 | 19.05 |
| 7/8 in | 0.8750 | 22.22 |
| 1 in | 1.0000 | 25.4 |
Common metric sizes in inches
| mm | Inches | Nearest 1/64 in |
|---|---|---|
| 3 mm | 0.118 | 1/8 |
| 4 mm | 0.157 | 5/32 |
| 5 mm | 0.197 | 13/64 |
| 6 mm | 0.236 | 15/64 |
| 7 mm | 0.276 | 9/32 |
| 8 mm | 0.315 | 5/16 |
| 10 mm | 0.394 | 25/64 |
| 11 mm | 0.433 | 7/16 |
| 12 mm | 0.472 | 15/32 |
| 13 mm | 0.512 | 33/64 |
| 14 mm | 0.551 | 35/64 |
| 15 mm | 0.591 | 19/32 |
| 16 mm | 0.630 | 5/8 |
| 17 mm | 0.669 | 43/64 |
| 18 mm | 0.709 | 45/64 |
| 19 mm | 0.748 | 3/4 |
| 22 mm | 0.866 | 55/64 |
| 24 mm | 0.945 | 15/16 |
The arithmetic explains some everyday near-misses between the two systems: 13 mm is 0.512 in, so a 13 mm wrench is within 0.3 mm of a 1/2 in one; 19 mm is 0.748 in, almost exactly 3/4 in; and 6 mm is 0.236 in, noticeably smaller than 1/4 in (6.35 mm). Whether a given tool is a safe substitute depends on its tolerances, so treat these as a guide and test the fit.
Tips
- When a drawing mixes units, convert everything to millimeters first; adding fractions by hand is where mistakes creep in.
- Paper, film and photo sizes are often metric in disguise: 35 mm film is 1.378 in wide, and A4 paper (210 × 297 mm) is 8.27 × 11.69 in.
- For larger lengths the cm to inches converter gives feet and inches, and the unit converter covers meters, yards and miles.
Frequently asked questions
How many mm are in an inch?
Exactly 25.4 millimeters. To convert millimeters to inches, divide by 25.4; to convert inches to millimeters, multiply by 25.4.
What is 10 mm in inches as a fraction?
10 mm is 0.3937 inches. The nearest common fraction is 25/64 in (9.92 mm); to the nearest sixteenth it rounds to 3/8 in, which is 0.48 mm smaller than 10 mm.
How do I type 5/16 inch into the converter?
Switch to Inches → millimeters and enter the fraction as a decimal: 5 ÷ 16 = 0.3125. The result is 7.9375 mm.
Is 1/64 inch bigger or smaller than a millimeter?
Smaller. 1/64 in = 0.397 mm, so there are about 2.5 sixty-fourths in a millimeter. 1/32 in is 0.79 mm and 1/16 in is 1.59 mm.
Why does the fraction row show a + or − value?
It is the rounding error: how much longer (+) or shorter (−) the nearest fraction is than the exact size, in millimeters. A value of 0.05 mm or less is usually invisible in woodworking; machining tolerances are often tighter.
Last updated: October 9, 2026