Instant Inch to Millimeter Calculator
Enter inches as a decimal (e.g., 2.75) or fraction (e.g., 1 1/2 or 3/8), then convert instantly.
How to Convert Inches to Millimeters
Converting inches to millimeters is simple because the relationship is exact: 1 inch = 25.4 millimeters. To convert, multiply your inch value by 25.4.
For example, if a part measures 3 inches wide, the metric equivalent is: 3 × 25.4 = 76.2 mm.
Why This Conversion Matters
Inch-to-millimeter conversion is common in engineering, manufacturing, woodworking, 3D printing, automotive work, and home improvement. Many tools and plans are imperial, while modern components and technical specifications are often metric.
- Machining: Tight tolerances are usually listed in millimeters.
- Construction: Imported materials and fixtures often use metric dimensions.
- DIY projects: Drill bit sizes, hardware, and templates may mix both systems.
- Technical documentation: CAD drawings commonly include metric dimensions.
Common Inch to Millimeter Conversions
| Inches | Millimeters |
|---|---|
| 1/8 in | 3.175 mm |
| 1/4 in | 6.35 mm |
| 3/8 in | 9.525 mm |
| 1/2 in | 12.7 mm |
| 3/4 in | 19.05 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 12 in | 304.8 mm |
Step-by-Step Conversion Method
1) Start with an inch measurement
Example: 5.5 in
2) Multiply by 25.4
5.5 × 25.4 = 139.7
3) Label the answer in millimeters
Final result: 139.7 mm
Tips for Accurate Results
- Use decimals for calculators and digital tools whenever possible.
- Keep extra decimal places during calculations, then round at the end.
- For precision work, confirm the required tolerance before rounding.
- If your input is fractional (like 7/16), convert correctly before multiplying.
Frequently Asked Questions
Is 25.4 exact or approximate?
It is exact. The inch is internationally defined as exactly 25.4 millimeters.
Can I enter fractions in this calculator?
Yes. You can enter mixed fractions like 1 1/2, simple fractions like 3/8, and standard decimals like 2.125.
How do I convert mm back to inches?
Use the reverse formula: inches = millimeters ÷ 25.4.