Authoritative definitions of the International System of Units and SI prefixes.
Unit conversion
Inches to Millimetres Converter
1 in = 25.4 mm. Enter a value to convert in to mm, then use the formula and table below. Displayed digits may be rounded.
Length & distanceInstant result
Decimals
Formula result = input × 0.0254 ÷ 0.001. 1 in = 25.4 mm
Nonzero results too small for the selected decimal places use scientific notation.
Nonzero results too small for the selected decimal places use scientific notation.
Common examples
One value, several units
Metres0.0254 m
Kilometres2.54000e-5 km
Centimetres2.54 cm
Millimetres25.4 mm
Miles1.57828e-5 mi
Yards0.0278 yd
Feet0.0833 ft
Nautical miles1.37149e-5 nmi
Conversion table
| in | mm |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 100 in | 2,540 mm |
Conversion chart
1 in25.4 mm
5 in127 mm
10 in254 mm
25 in635 mm
50 in1,270 mm
Worked example
1 in = 25.4 mm. Apply the selected conversion formula to other input values.
Unit definition
Inches (in) One inches equals 0.0254 metre.
Unit history
Inches is represented here using its modern SI definition or an internationally accepted relationship to an SI base unit.
Step by step
1. Parse the value, including fractions.
2. Convert in to metre.
3. Convert the base value to mm and apply your display precision.
Understand the result before using it
Inches to Millimetres converts a value measured in Inches (in) into the equivalent value in Millimetres (mm). The numerical value changes because the size and definition of the selected unit changes; the underlying quantity does not.
Calculated result
The conversion follows the displayed unit relationship. Display rounding can hide additional digits, and measured inputs cannot become more accurate merely by changing units.
Formula, assumptions, and example
Formula
result = input × 0.0254 ÷ 0.001
Assumptions
- Inches and Millimetres represent the same length & distance quantity.
- The input is expressed in in; the output is expressed in mm.
- The unit definitions and conventional relationship shown on this page apply.
Worked example
1 in = 25.4 mm. For another value, apply the same relationship and round only to the precision your source measurement supports.
When to use it
Good uses
- Read a in measurement in a document that expects mm.
- Compare a inches value with a value reported in millimetres.
- Check the in to mm arithmetic before copying the result into another calculation.
Limitations
Common mistakes to avoid
- Do not confuse in with another unit that has a similar name or symbol.
- Round the output to a precision justified by the original measurement.
- Verify safety-critical, regulated, laboratory, and contractual conversions against the controlling standard.
Sources and further reading
These references explain the underlying standards or provide authoritative context. Your own contract, institution, clinician, product documentation, local code, or governing standard may be the controlling source.
Guidance on unit names, symbols, conversions, and numerical presentation.
Questions about this calculation
How do I convert Inches to Millimetres?
result = input × 0.0254 ÷ 0.001. Input is in in; result is in mm. 1 in = 25.4 mm. The displayed digits may be rounded.
What is 1 in in mm?
1 in = 25.4 mm. The displayed digits may be rounded.
Can I reverse the in to mm conversion?
Yes. Swap the source and target units in the converter. The reverse relationship converts mm back to in.
Is the Inches to Millimetres result exact?
The conversion follows the displayed unit relationship. Display rounding can hide additional digits, and measured inputs cannot become more accurate merely by changing units.