Convert inches to millimeters instantly. Type a value and the result updates as you go — converting US fastener and pipe sizes to metric.
How to convert inches to millimeters
Multiply the inches amount by 25.4. For example, 10 in × 25.4 = 254 mm.
Quick in to mm reference
A few everyday values so you don’t have to reach for the calculator:
| in | mm |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 50 in | 1270 mm |
| 100 in | 2540 mm |
| 250 in | 6350 mm |
| 500 in | 12700 mm |
| 1000 in | 25400 mm |
Why convert inches to millimeters
People reach for this conversion converting US fastener and pipe sizes to metric. Having it computed in the browser means no upload, no wait and a result you can trust to the decimal shown.
Frequently asked questions
How many millimeters are in one inch?
Exactly 25.4 mm. In other words, 1 in = 25.4 mm.
How do I convert inches to millimeters?
Multiply the inches amount by 25.4. For example, 10 in × 25.4 = 254 mm.
How do I convert millimeters back to inches?
Divide the millimeters amount by 25.4 (or multiply by 0.03937007874). 10 mm ÷ 25.4 = 0.3937007874 in.
inches vs millimeters: which is bigger?
One inch is larger than one millimeter: 1 in = 25.4 mm. You need fewer millimeters to express the same amount.
How to use it
- Type a number into the inches input field.
- Watch the millimeters result update in real time as you type.
- Use the values in the quick reference table below if your number is already there — no input needed.
- Copy the result and paste it into a parts list, CAD drawing or specification sheet.
Where it comes in handy
A mechanical drawing specifies a bolt at 3/8 in. The metric equivalent is 9.525 mm, so you order an M10 fastener instead — the closest standard size.
A recipe calls for a baking pan that is 13 in by 9 in. Converting to 330 mm by 229 mm tells you whether it fits in a European oven with a 60 cm cavity.
A US pipe is labeled 2 in nominal diameter. The outside diameter is actually 2.375 in, which converts to 60.3 mm — critical when checking whether it fits through a metric hole.
Worth knowing
- The conversion factor 25.4 is exact, so no precision is lost — but the result may show more decimal places than your source data justifies.
- Rounding matters when specifying fasteners: 7.9 mm rounds to 8 mm for small hardware but to 7.9 mm for precision shim stock where 0.1 mm is the tolerance band.
- Pipe sizes are misleading — a 1/2 in NPS pipe has an outside diameter of 0.840 in (21.3 mm), not 12.7 mm, because the inch designation refers to rough bore, not OD.