Length conversion
Convert Millimeters to Inches
1 millimeter equals 0.03937 inches. To convert millimeters to inches, divide the millimeters by 25.4 (or multiply by 0.03937).
Millimeters to inches is the maker's conversion — fastener sizes, 3D-printer dimensions, wrench and drill-bit specs, and phone-screen measurements all show up in mm when your tools are imperial. Enter any millimeter value for an instant inch result with the exact factor shown.
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How to convert millimeters to inches
Formula
inches = millimeters ÷ 25.4 millimeters = inches × 25.4Because one inch is exactly 25.4 millimeters (2.54 cm), this conversion is exact. A handy reference: 1 mm ≈ 0.039 in, and 25.4 mm = 1 in.
| Millimeters | Inches |
|---|---|
| 1 mm | 0.03937 in |
| 2 mm | 0.07874 in |
| 3 mm | 0.1181 in |
| 5 mm | 0.1969 in |
| 10 mm | 0.3937 in 1 cm |
| 20 mm | 0.7874 in |
| 25 mm | 0.9843 in |
| 25.4 mm | 1 in exactly 1 inch |
| 50 mm | 1.969 in |
| 100 mm | 3.937 in |
Worked example
A bolt is marked M8 with a length of 40 mm and you need the inch equivalent for an imperial chart. Divide 40 by 25.4 to get about 1.575 inches, just over 1½ inches. For thin stock, a 3 mm sheet is 0.118 inches — close to the 1/8-inch (0.125 in) imperial size but not identical, which matters for tight tolerances.
Shortcut. Rule of thumb: 25 mm is almost exactly one inch, and 1 mm is roughly 1/25 of an inch (0.04 in). For finer work, remember 0.5 mm ≈ 0.020 in.
Good to know
Millimeters are the go-to unit for precision because they avoid fractions, so converting to inches often means swapping a clean metric number for a decimal or the nearest imperial fraction. When matching hardware, compare the decimal inch value to fractional sizes (1/16 in = 0.0625 in, 1/8 in = 0.125 in) rather than assuming a metric and imperial part interchange.
Frequently asked questions
How many inches is 1 mm?
One millimeter is 0.03937 inches — about four-hundredths of an inch. Divide any millimeter value by 25.4 to convert to inches.
What is 10 mm in inches?
10 mm (one centimeter) equals 0.3937 inches. 5 mm is 0.1969 in and 20 mm is 0.7874 in.
How many mm are in an inch?
Exactly 25.4 millimeters. That makes 25.4 mm the cleanest reference point when converting either direction.
Is 3 mm the same as 1/8 inch?
Not quite. 3 mm is 0.118 inches while 1/8 inch is 0.125 inches, so the imperial size is slightly larger — enough to matter for press fits and drilling.
How do I convert mm to inches for 3D printing?
Divide the millimeter dimension by 25.4. Most slicers work in mm natively, so you usually only convert when comparing to an imperial part or hardware spec.