Instant Unit Conversion
Convert Inches to Millimeters (in to mm)
Exact formula, conversion factors, live calculator, and reference charts for inches to millimeters.
Method reviewed:Exact defined factors — international yard and pound (1959), US customary gallon and cup, SI — the values NIST SP 811 lists · reviewed September 2026
Direct AnswerFormula: mm = in × 25.4
1 inch is equal to exactly 25.4 millimeters. To convert inches to millimetres, multiply by 25.4.
Live Inches to Millimeters Calculator
Formula:
mm = in × 25.4Open full length converter →Inches to Millimeters Conversion Table
Quick reference conversion chart from 1 to 100 inches.
| Inches (in) | Millimeters (mm) | Mathematical Formula |
|---|---|---|
| 1 in | 25.4 mm | 1 × 25.4 |
| 2 in | 50.8 mm | 2 × 25.4 |
| 5 in | 127 mm | 5 × 25.4 |
| 10 in | 254 mm | 10 × 25.4 |
| 15 in | 381 mm | 15 × 25.4 |
| 20 in | 508 mm | 20 × 25.4 |
| 25 in | 635 mm | 25 × 25.4 |
| 50 in | 1,270 mm | 50 × 25.4 |
| 75 in | 1,905 mm | 75 × 25.4 |
| 100 in | 2,540 mm | 100 × 25.4 |
Frequently Asked Questions
How many mm is 1/2 inch?
Half an inch is exactly 12.7 mm.
Why is 25.4 exact?
The 1959 international yard and pound agreement defined the yard as exactly 0.9144 m, which makes the inch exactly 25.4 mm with no rounding anywhere.
What is 3/8 inch in mm?
3/8 inch is 9.525 mm — commonly rounded to 9.5 mm on a metric drill index.