Enter measurement details
Formula used
One inch equals exactly 25.4 millimetres. Multiply inches by 25.4 for conversion. Divide millimetres by 25.4 to calculate accurate inch values.
Millimetres = Inches × 25.4 Inches = Millimetres ÷ 25.4
How to use this calculator
- Select the required conversion direction.
- Choose decimal, fraction, or multiple values.
- Enter measurements and select decimal precision.
- Choose the preferred rounding method.
- Press the conversion button for detailed results.
- Copy or download the completed conversion report.
Worked examples
| Inches | Calculation | Millimetres |
|---|---|---|
| 1/16 | 0.0625 × 25.4 | 1.5875 mm |
| 1/8 | 0.125 × 25.4 | 3.175 mm |
| 1/4 | 0.25 × 25.4 | 6.35 mm |
| 1/2 | 0.5 × 25.4 | 12.7 mm |
| 1 | 1 × 25.4 | 25.4 mm |
| 2 | 2 × 25.4 | 50.8 mm |
| 10 | 10 × 25.4 | 254 mm |
Recent calculation history
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Frequently asked questions
How many millimetres equal one inch?
One inch equals exactly 25.4 millimetres. This value is internationally defined. It remains consistent across engineering, manufacturing, and household measurements.
How do I convert inches to millimetres?
Multiply the inch value by 25.4. Keep enough decimal places for accuracy. Round only after completing every required conversion calculation carefully.
How do I convert millimetres to inches?
Divide the millimetre value by 25.4. The result may contain repeating decimals. Choose precision that matches your intended measurement purpose best.
Can this calculator use fractions?
Yes, it accepts common mixed fractions. Enter whole, numerator, and denominator values. Preset buttons make frequent fractional measurements faster to enter.
Can I convert several values together?
Yes, choose the multiple values option. Separate entries using commas or lines. Each valid measurement receives its own clearly labelled result.
Why does rounding change the displayed answer?
Rounding shortens long decimal results. The exact result remains available separately. The displayed difference reveals how rounding changed your answer.
Which rounding method should I choose?
Standard rounding suits most general measurements. Upward rounding supports minimum size requirements. Truncation removes extra digits without changing earlier decimal places.
Are negative measurements supported?
Yes, decimal inputs may be negative. Negative values can represent coordinates or offsets. Physical lengths normally remain positive during practical measurement work.
How accurate is the conversion?
The conversion factor is exact. Display accuracy depends on selected precision. Source measurement quality still determines the final practical accuracy.
Can I download my conversion results?
Yes, download results as CSV files. PDF reports are also available. Printing can preserve the current page for offline reference.
Does calculation history leave my browser?
No, history uses local browser storage. The server does not receive saved history. Clearing browser data permanently removes all stored calculation entries.