Inches to Millimeters Calculator
Enter a value in inches and instantly convert to millimeters (mm). You can type decimals (2.75) or fractions (3/8, 1 1/2).
How to convert inches to millimeters
To convert inches to millimeters, multiply the inches value by 25.4. This is an exact conversion factor because one inch is internationally defined as exactly 25.4 millimeters.
Conversion formula
millimeters (mm) = inches (in) × 25.4
- 1 in = 25.4 mm
- 2 in = 50.8 mm
- 0.5 in = 12.7 mm
Why this conversion matters
Inches are common in the United States for construction, tools, and hardware. Millimeters are widely used in engineering, manufacturing, 3D printing, and international technical documentation. If you work with both systems, quick and accurate conversion avoids expensive mistakes.
- Engineering drawings: match dimensions from imperial to metric specs.
- DIY and woodworking: use imported plans and tools correctly.
- Automotive and mechanics: compare part tolerances from different standards.
- 3D printing and CNC: avoid scaling errors in model files.
Manual examples
Example 1: Convert 3 inches to mm
3 × 25.4 = 76.2 mm
Example 2: Convert 7.25 inches to mm
7.25 × 25.4 = 184.15 mm
Example 3: Convert a fraction (3/8 inch)
First convert fraction to decimal: 3/8 = 0.375. Then 0.375 × 25.4 = 9.525 mm.
Quick reference chart: inches to mm
| Inches | Millimeters (mm) |
|---|---|
| 1/16 in | 1.5875 mm |
| 1/8 in | 3.175 mm |
| 1/4 in | 6.35 mm |
| 1/2 in | 12.7 mm |
| 3/4 in | 19.05 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 4 in | 101.6 mm |
| 6 in | 152.4 mm |
| 12 in (1 ft) | 304.8 mm |
Tips for accurate measurements
- Use a quality ruler, caliper, or tape with clear markings.
- Always check whether your source value is decimal inches or fractional inches.
- Round only at the final step if precision matters.
- For machining and design, match the decimal precision required by your project.
Frequently asked questions
Is 1 inch exactly 25.4 mm?
Yes. This is an exact international standard, not an estimate.
Can I enter fractions in this calculator?
Yes. You can input values like 1/8, 3/16, or 1 1/2, and the calculator will convert them automatically.
How many mm are in 10 inches?
10 × 25.4 = 254 mm.