Length conversion — instant, free, runs in your browser.
How to convert inches to millimeters
To convert inches to millimeters, multiply the value in in by 25.4 — mm = in × 25.4. The calculator
above does this instantly for any value, in both directions — use Swap units to reverse it.
Inches to Millimeters conversion table
| Inches (in) | Millimeters (mm) |
|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 10 | 254 |
| 15 | 381 |
| 20 | 508 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
Related conversions
Need the opposite direction? Millimeters to Inches.
Every length pair lives on the length converter page,
and the full unit converter covers six other categories.
Convert inches to other units
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Common mistakes to avoid
- Mixing up ounces. The Weight category uses the ounce (oz), a unit of mass (28.3495 g). The Volume category uses the US fluid ounce (fl oz), a unit of capacity (29.5735 mL). One fluid ounce of water weighs about 1.04 oz, not exactly 1 oz, so always confirm which category you are in before converting.
- Confusing bits and bytes. File sizes are measured in bytes, network speeds in bits. In Data storage, 1 byte equals 8 bits, so a 100 Mbit per second connection transfers at most 12.5 megabytes per second.
- Assuming imperial gallons. Every volume unit here is US customary: 1 US gallon is 3.785 liters. A UK imperial gallon is 4.546 liters, roughly 20 percent larger, so results will not match UK fuel economy figures.
A quick accuracy check
Want to sanity-check any converter? Convert -40 from Celsius to Fahrenheit: the result must be exactly -40, the only temperature where the two scales meet. Then convert 0 Kelvin to Celsius: you should get -273.15, absolute zero. A converter that handles both edge cases correctly has sound temperature formulas, and this one does.
Frequently asked questions
How many millimeters are in one inche?
One inche is exactly 25.4 mm. The table above lists the converted values for common amounts from 1 to 100.
How do I convert inches to millimeters?
Use the converter above — type any value and the result updates instantly — or do the math yourself: multiply the value in in by 25.4 — mm = in × 25.4.