Convert Inches to Millimeters
1 inch is 25.4 mm. One inch is exactly 25.4 millimeters.
One inch is 25.4 millimeters. To convert inches to millimeters, multiply by 25.4; to go the other way, divide by 25.4 or multiply by 0.039370079. The factor is exact rather than rounded: it follows from NIST. Exactly 25.4 millimeters since the 1959 international yard and pound agreement, and one twelfth of a foot.
Factor checked against the defining standard · How we check
- Inches × 25.4
1 * 25.425.4 mm
1 inch in every length unit
| Unit | Value | Exact factor |
|---|---|---|
| inches (as entered) | 1 in | — |
| feet | 0.08333333333 ft | × 0.0833333333333 |
| yards | 0.02777777778 yd | × 0.0277777777778 |
| miles | 0.00001578282828 mi | × 0.0000157828282828 |
| millimeters | 25.4 mm | × 25.4 |
| decimetres | 0.254 dm | × 0.254 |
| centimeters | 2.54 cm | × 2.54 |
| meters | 0.0254 m | × 0.0254 |
| kilometers | 0.0000254 km | × 0.0000254 |
| nautical miles | 0.00001371490281 nmi | × 0.0000137149028078 |
| micrometers | 25,400 µm | × 25,400 |
| nanometers | 25,400,000 nm | × 25,400,000 |
| thou | 1,000 thou | × 1,000 |
Factors are entered by hand from NIST rather than derived from a units library, so the ones that terminate are exact rather than rounded.
The same converter answers this pair however it is written: inches to millimetres is the British spelling of the same units — the metre and the litre are spelled that way outside the United States and the quantity does not change.
How do you convert inches to millimeters? The converter and the working
- 1.Take the measurement in inches.
- 2.Multiply by 25.4 — that is how many millimeters fit in one inch.
- 3.To go back the other way, divide by 25.4, or multiply by 0.039370079.
Worked example: 25 inches to millimeters
- Formula
- in × 25.4 = mm
- Arithmetic
- 25 × 25.4 = 635 mm
- Answer
- 25 in = 635 mm
The factor is exact, so this figure is exact too — only the display is rounded.
Inches to millimeters conversion table
| Inches | Millimeters |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 100 in | 2,540 mm |
| 200 in | 5,080 mm |
| 300 in | 7,620 mm |
| 500 in | 12,700 mm |
| 1000 in | 25,400 mm |
| 2000 in | 50,800 mm |
| 5000 in | 127,000 mm |
| 10000 in | 254,000 mm |
What is an inch?
Exactly 25.4 millimeters since the 1959 international yard and pound agreement, and one twelfth of a foot.
What is a millimeter?
One thousandth of a meter. The working unit of metric engineering drawings, which avoid decimals by using it throughout.
How many millimeters are in an inch?
One inch is 25.4 millimeters, exactly. Going the other way, one millimeter is 0.039370079 inches. Both figures come from the same defined relation rather than from a measurement, which is why they are the same today as they were in 1959.
Every length unit here
The whole group — 13 units with their symbols and the size of each one in meters — is on the conversion index, along with every pair it makes. The table above already gives each of them relative to an inch.
Source: NIST — international yard and pound agreement (inch = 25.4 mm exactly)
Cite this page
- APA
- Rule Calculator. (2026). Convert Inches to millimeters. Rule Calculator. https://rulecalculators.com/convert/inch-to-millimeter
- MLA
- "Convert Inches to millimeters." Rule Calculator, August 16, 2026, https://rulecalculators.com/convert/inch-to-millimeter.
- Chicago
- Rule Calculator. "Convert Inches to millimeters." Rule Calculator. Last reviewed August 16, 2026. https://rulecalculators.com/convert/inch-to-millimeter.
The date is when the figures on this page were last checked against their source, not the day you opened it. Add your own access date if your style needs one.
Questions people actually ask
- How many millimeters are in an inch?
- 25.4. One thousandth of a meter. The working unit of metric engineering drawings, which avoid decimals by using it throughout.
- How do I convert millimeters back to inches?
- Divide by 25.4, or multiply by 0.039370079 — they are the same operation.
Other length conversions
Millimeters to inches — the same conversion the other way.