1 inch equals 25.4 millimetres.
The inch is a unit of length in the imperial system, and the millimetre is a metric unit. To convert inches to millimetres, you multiply the number of inches by 25.4 because one inch is defined as exactly 25.4 millimetres.
Conversion Tool
Result in millimetres:
Conversion Formula
The conversion from inch to millimetres uses the formula:
millimetres = inches × 25.4
This works because the inch is legally defined as exactly 25.4 millimetres, so multiplying the inch value by 25.4 converts it directly into millimetres. The factor 25.4 acts as the scaling between the two units.
For example, if you want to convert 1.5 inches:
- Multiply 1.5 (inches) by 25.4
- 1.5 × 25.4 = 38.1 millimetres
- So, 1.5 inches equals 38.1 millimetres
Conversion Example
- Convert 3 inches to millimetres:
- 3 × 25.4 = 76.2 mm
- Each inch corresponds to 25.4 millimetres, so 3 inches is 76.2 millimetres.
- Convert 0.75 inches to millimetres:
- 0.75 × 25.4 = 19.05 mm
- Multiply the fractional inch value by 25.4 to get millimetres.
- Convert 5.5 inches to millimetres:
- 5.5 × 25.4 = 139.7 mm
- This converts a larger inch value into millimetres by simple multiplication.
- Convert 2.25 inches to millimetres:
- 2.25 × 25.4 = 57.15 mm
- Multiply to get the precise length in millimetres.
Conversion Chart
This chart shows inch values from -24.0 to 26.0 and their millimetre equivalents. Negative values represent lengths in the opposite direction or below zero, while positive values indicate normal lengths. Use the chart by finding the inch value and reading across to millimetres.
| Inches | Millimetres (mm) |
|---|---|
| -24.0 | -609.6 |
| -20.0 | -508.0 |
| -16.0 | -406.4 |
| -12.0 | -304.8 |
| -8.0 | -203.2 |
| -4.0 | -101.6 |
| 0.0 | 0.0 |
| 2.0 | 50.8 |
| 4.0 | 101.6 |
| 6.0 | 152.4 |
| 8.0 | 203.2 |
| 10.0 | 254.0 |
| 12.0 | 304.8 |
| 14.0 | 355.6 |
| 16.0 | 406.4 |
| 18.0 | 457.2 |
| 20.0 | 508.0 |
| 22.0 | 558.8 |
| 24.0 | 609.6 |
| 26.0 | 660.4 |
Related Conversion Questions
- How many millimetres are there in 1 inch exactly?
- Is 1 inch equal to 25 millimetres or 25.4 millimetres?
- What’s the quickest way to convert 1 inch to millimetres?
- Can you convert 1 inch to millimetres without a calculator?
- Why does 1 inch equal 25.4 millimetres in measurement?
- How precise is the 1 inch to millimetres conversion?
- What tools can convert 1 inch measurement into millimetres?
Conversion Definitions
Inch: The inch is a unit of length in the imperial system, defined as exactly 25.4 millimetres. It’s used mainly in the United States and the UK for measuring smaller distances, such as in construction, manufacturing and everyday objects. One foot equals 12 inches.
Millimetres: The millimetre is a metric unit of length equal to one thousandth of a meter. It’s commonly used worldwide for precise measurements in engineering, science, and manufacturing. Millimetres allow for very fine detail when measuring small objects or distances.
Conversion FAQs
Is the conversion factor 25.4 always exact?
Yes, the inch is internationally defined as exactly 25.4 millimetres since 1959 by agreement between the US and countries using the metric system. This means the conversion factor is not an approximation, but a fixed constant used worldwide.
Why do some people approximate 1 inch as 25 mm?
In casual or quick estimations, people sometimes round 25.4 mm down to 25 mm for simplicity. However, this introduces a small error that might be acceptable for rough measurements but not precise work.
Can I convert millimetres back to inches using the same factor?
Yes, to convert millimetres to inches, divide the millimetre value by 25.4. This reverses the multiplication done when converting inches to millimetres, ensuring accurate two-way conversion.
Does temperature affect inch to millimetres conversion?
No, the conversion factor is a fixed length relationship and does not change with temperature. However, materials being measured may expand or contract with temperature changes, affecting actual length but not the unit conversion itself.
Are inches and millimetres used together in any industries?
Yes, some industries like manufacturing and automotive use both units. For example, parts designed in metric countries might be specified in millimetres, but assembled or used in countries where inches are common, requiring conversion.