Inch to Millimetre Converter
Convert inches to mm instantly. Free inches to mm calculator with formula, examples and FAQs.
Quick answer
1 inches equals 25.4 mm. To convert inches to mm, multiply the inches value by 25.4 — so 10 inches = 254 mm. This factor is exact, and the reverse conversion (mm to inches) simply divides by 25.4 instead of multiplying.
Converted value
25.40
1.00 inches = 25.40 mm
What is inches to mm?
Inch and Millimetre both measure length, so converting inches to mm restates the same quantity in a different unit. Online shoppers and DIY buyers reach for this conversion regularly. The relationship is fixed and linear, which means one factor (mm = inches × 25.4) maps every inches value to its exact mm equivalent.
Real-world uses: why convert inches to mm?
Converting inches to mm bridges two ways of measuring the same thing. People do it most often when reading US product dimensions for shipping, and on the receiving side when checking jewellery, screw and component sizes. Having the exact value — not a rough estimate — prevents costly mistakes in paperwork, purchases and planning.
- •Converting screen and TV sizes to metric.
- •Reading US product dimensions for shipping.
- •Matching tyre, pipe and hardware sizes.
- •Understanding inch measurements on rulers.
- •Checking jewellery, screw and component sizes.
Who uses this conversion?
How the inches to mm conversion is calculated
This is the same arithmetic that calculators and search engines use — no rounded shortcuts.
mm = inches × 25.4
- Both inches and mm are defined in terms of the same base unit, the metre.
- 1 inches = 0.0254 metre, and 1 mm = 0.001 metre.
- Divide one definition by the other to get the conversion factor: 0.0254 ÷ 0.001 = 25.4.
- So mm = inches × 25.4. Multiply any inches value by 25.4 to get the result in mm.
Precision: The conversion factor is exact by international definition, so the result is an exact rational number. We display it rounded to four decimal places (or four significant figures for very small values).
Example calculations
- •1 inches = 25.4 mm
- •6 inches = 152.4 mm
- •12 inches = 304.8 mm
inches to mm quick reference table
Common inches values converted to mm, each calculated with the exact factor.
| inches | mm |
|---|---|
| 10 | 254 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
| 250 | 6350 |
| 500 | 12700 |
| 1000 | 25400 |
| 2500 | 63500 |
| 5000 | 127000 |
| 10000 | 254000 |
Common mistakes to avoid
- •Reading the result the wrong way round. Confirm the answer is in mm, not inches, before you rely on it.
- •Truncating instead of rounding, which compounds error across larger inches values.
- •Mixing metric and imperial units midway through a multi-step conversion instead of converting to a single base unit first.
Common questions about length conversions
Is this length conversion exact?
Yes. Factors such as 1 inch = 25.4 mm and 1 mile = 1,609.344 m are exact by international agreement, so the only rounding is in how many decimals we display.
Why do my results differ slightly from another tool?
Almost always because the other tool rounds the factor (e.g. 0.62 instead of 0.621371) or rounds partway through. We keep full precision and round only the final value.
Metric or imperial — which should I use?
Use metric (m, cm, km) for science and most countries, and imperial (ft, in, miles) for US and UK everyday distances. This page bridges the two exactly.
Can I round the answer?
Yes. For everyday use one or two decimals is plenty; engineering and science may need more. Round only at the end to avoid cumulative error.
History of these units
History of the inch
imperial / US customary
The inch traditionally matched the width of a thumb and was later set at 1/12 of a foot. Since 1959 it is exactly 25.4 millimetres and remains common for screen sizes, hardware and US dimensions.
History of the millimetre
metric
The millimetre is one-thousandth of a metre. As metric precision spread through engineering in the 19th and 20th centuries, it became the default unit for technical drawings and machined tolerances.
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