Length conversion
Inches to Millimetres (in to mm)
Convert decimal inches on a drawing or measuring tool to millimetres. The input is a measured length, so any nominal size needs checking against the product’s dimensions.
1 in = 25.4 mm — the calculator keeps the stored factor; this summary may be rounded.
A measured thickness of 1 inch
- Starting measurement
- 1 in
- Converted value (rounded if needed)
- 25.4 mm
For half an inch, enter 0.5. Keep any tolerance from the original drawing; conversion does not make an approximate measurement exact.
Inches to Millimetres conversion chart
| inches | millimetres |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 15 in | 381 mm |
| 18 in | 457.2 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 30 in | 762 mm |
| 40 in | 1016 mm |
| 50 in | 1270 mm |
| 60 in | 1524 mm |
| 75 in | 1905 mm |
| 100 in | 2540 mm |
How to convert by hand
Reference relationship (display may be rounded)
1 in = 25.4 mm
- Multiply the amount in inches by the number of millimetres per unit.
- Use the calculator for the stored precision; copying a short displayed factor introduces extra rounding.
- Round the result to suit your measurement, after completing the calculation.
Related conversions
About inches and millimetres
Convert each measured dimension separately. A width stays a width and a diagonal stays a diagonal; changing units does not turn a product’s nominal size into a measured size.
Inch
US / ImperialExactly 2.54 centimetres under the 1959 international yard agreement. The symbol is in; a double-prime mark is also used on drawings.
Commonly used for: Screen diagonals, picture sizes and dimensions on US product listings.
Millimetre
Metric (SI)One thousandth of a metre. Small differences matter when checking a part against a drawing.
Commonly used for: Material thickness, fastener dimensions and workshop measurements.
Questions
Does a product sold as one inch always measure 25.4 mm?
One measured inch is exactly 25.4 mm. A nominal product size, such as a trade size for timber or pipe, can describe something other than its finished measured dimension. Use the specification’s actual dimension.
How do I convert millimetres back to inches?
Enter the value in the millimetres field above to work backwards, or open the Millimetres to Inches converter. This uses the stored factors rather than dividing by a rounded number copied from the chart.
Why might the chart and calculator show different decimal places?
The chart uses a compact rounded display. The editable calculator can show up to six decimal places and uses the factors or temperature formulas stored in the library. More displayed digits do not make the original measurement more precise.
Precision and source
The calculator uses stored conversion factors and temperature formulas, then rounds the result for display. A short decimal in a chart is not a claim of exactness. For a calculation with several steps, keep as much precision as the original measurement supports and round at the end.
Reference: NIST SP 811 conversion factors. Its alphabetical tables distinguish unit variants and temperature readings from intervals. Published by ToolsNow. Report a conversion problem.
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