How to convert inches to millimeters
Every inch is 25.4 millimeters, so the conversion is a single multiplication:
Worked example: 25 in → 25 × 25.4 = 635 mm.
Quick mental estimate: Multiply by 25: 25 × 25 ≈ 625 mm (exact: 635).
Inches to millimeters conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 75 in | 1,905 mm |
| 100 in | 2,540 mm |
| 200 in | 5,080 mm |
| 250 in | 6,350 mm |
| 500 in | 12,700 mm |
| 1,000 in | 25,400 mm |
Values rounded to four decimals. For a value that is not in the table, use the converter above — it accepts decimals and negative numbers.
When you need this conversion
- Buying anything sized abroad: screens, bike frames, luggage limits and clothing patterns switch between metric and imperial depending on where they were made.
- DIY and construction: timber, pipe diameters and fixings are stocked in one system or the other, and a plan drawn in one has to be ordered in the second.
- Travel and sport: race distances, altitudes and road signs change unit at almost every border.
Getting it right
The arithmetic is a single multiplication, so mistakes almost never come from the calculation itself — they come from the value that goes into it. Check that the figure you are converting really is in inches and not in a neighbouring unit, and that you have not already converted it once.
Sanity-check the direction of the result before you use it. One inch is 25.4 millimeters, so converting inches to millimeters should make the number larger. If it moved the other way, the conversion was applied backwards — the reverse converter is one click away.
About the units
Inch (in)
The inch is one twelfth of a foot, fixed at exactly 2.54 centimeters by international agreement in 1959. Screens, paper, pipes and construction lumber in the US are all sized in inches.
Millimeter (mm)
The millimeter is one thousandth of a meter. Engineering drawings, machine parts, rainfall and screen sizes are specified in millimeters because the unit is fine enough to avoid decimals.
Frequently asked questions
How many millimeters are in 1 inch?
1 inch equals 25.4 millimeters. To convert any value, multiply the number of inches by 25.4.
How do I convert inches to millimeters in my head?
Multiply by 25: 25 × 25 ≈ 625 mm (exact: 635).
Is the in to mm conversion exact?
1 inch is exactly 25.4 mm, so this conversion is exact.
How do I convert inches to millimeters in Excel or Google Sheets?
Put the value in A1 and use =A1*25.4. Both Excel and Google Sheets also have a built-in function, =CONVERT(A1,"in","mm"), which avoids typing the factor by hand.
How many decimal places should I keep?
For everyday use two decimals are plenty. Keep four if the result feeds another calculation, and round only at the very end — rounding intermediate steps is the most common reason a result comes out slightly wrong.