Convert Millimeters to Inches
Convert millimeters to inches by dividing by 25.4 — drop any value into the converter above and it handles everything from A4 paper to screw lengths instantly.
- Millimeter (mm)1
- Centimeter (cm)0.1
- Meter (m)0.001
- Kilometer (km)0.000001
- Inch (in)0.03937
- Foot (ft)0.003281
- Yard (yd)0.001094
- Mile (mi)0.000001
- Ja/Chuk (자)0.0033
- Chi/Chon (치)0.033003
🔒 Unit conversions are computed in your browser.
The formula and why it works
One inch has been defined as exactly 25.4 mm since the 1959 international yard-and-pound agreement. So converting millimeters to inches is simply mm ÷ 25.4, or written as multiplication, mm × 0.03937 (since 1/25.4 = 0.0393700787…). For example, 100 mm equals 100 ÷ 25.4 = 3.937 inches. Unlike temperature conversions, length is a pure proportional relationship — 0 mm is 0 inches — so there is no offset to add or subtract. Every value scales by the same ratio, which means you only ever multiply or divide.
Worked examples you can check
Real numbers make it click. A sheet of A4 paper is 210 × 297 mm; dividing each side by 25.4 gives 8.27 × 11.69 inches. A 25 mm bolt works out to 25 ÷ 25.4 = 0.984 inch — tantalizingly close to an inch, but a true inch is 25.4 mm. And an 18 mm sheet of plywood is 0.709 inch, which sits about 1 mm shy of the US nominal 3/4 inch (19.05 mm), so the two are not interchangeable on a cut list.
Rounding and sizing gotchas
A few traps to watch. Because 0.03937 is rounded, multiplying it by 1,000 mm yields 39.37 inches, whereas dividing precisely gives 39.3701 inches — the error grows with length, so divide by 25.4 when it matters. US woodworking and machine drawings also use fractional inches (1/16, 1/32) rather than decimals, so 0.709 inch rounds to roughly 11/16 (0.6875). Finally, the old US survey inch differed by a hair, but it was retired at the end of 2022, leaving 25.4 mm as the single standard.
Frequently asked questions
How many inches is 1 mm?
1 mm = 1 ÷ 25.4 = 0.03937 inch. Going the other way, 1 inch is exactly 25.4 mm, so to convert inches back to millimeters you multiply by 25.4.
Should I divide by 25.4 or multiply by 0.03937?
They are the same operation, because an inch is defined as exactly 25.4 mm. But 0.03937 is rounded at the fifth decimal place, so it introduces a tiny error on large values — 1,000 mm becomes 39.37 in by multiplication versus 39.3701 in by division. When precision matters, always divide by 25.4.
How do I convert mm to a fractional inch like 1/16?
Convert to a decimal inch first, then multiply by 16 (or 32 or 64) and round to get the numerator. For example, 10 mm = 0.3937 inch → 0.3937 × 16 = 6.3 → about 6/16, which simplifies to 3/8 inch (0.375). Use 32nds or 64ths for finer, more accurate results.