How Many Millimeters in an Inch?
There are exactly 25.4 millimeters in one inch — an exact international standard.
Last updated: 2026-03-15
The Quick Answer
There are exactly 25.4 millimeters in one inch. This is the precise, internationally agreed-upon definition.
Conversion Formulas
- Inches to mm: mm = Inches × 25.4
- mm to inches: Inches = mm ÷ 25.4
Common Fractional Inch to MM Conversions
| Inches (fraction) | Inches (decimal) | Millimeters |
|---|---|---|
| 1/16" | 0.0625 | 1.588 |
| 1/8" | 0.125 | 3.175 |
| 3/16" | 0.1875 | 4.763 |
| 1/4" | 0.25 | 6.35 |
| 3/8" | 0.375 | 9.525 |
| 1/2" | 0.5 | 12.7 |
| 5/8" | 0.625 | 15.875 |
| 3/4" | 0.75 | 19.05 |
| 7/8" | 0.875 | 22.225 |
| 1" | 1.0 | 25.4 |
Where This Comes Up
Engineering and Machining
CNC machining, engineering drawings, and precision manufacturing frequently require converting between inches (common in US shops) and millimeters (the international standard). Tolerance specifications of ±0.001" equal ±0.0254 mm.
Drill Bits and Fasteners
Drill bits are sold in both imperial and metric sizes. A 1/4" drill bit is 6.35 mm, while a 6 mm bit is 0.236". Having the conversion memorized prevents using the wrong size.
Screen and Display Technology
Screen sizes are measured in inches diagonally, but pixel density is sometimes expressed per mm. Converting between the two systems is routine in display engineering.
Key Fact
The 25.4 mm per inch relationship is the foundation of all imperial-to-metric length conversions. From this single definition, all other conversions (feet to meters, yards to meters, miles to kilometers) are derived. Use our length converter for any conversion.