1 inch is equal to 25.4 millimetres.
The inch is a unit of length traditionally used in the United States and the United Kingdom, while millimetres is part of the metric system. Converting 1 inch to millimetres involves multiplying by 25.4, since one inch has been internationally defined as exactly 25.4 millimetres.
Conversion Tool
Result in millimetres:
Conversion Formula
The conversion formula to change inches into millimetres is:
millimetres = inches × 25.4
This works because the international agreement defined one inch as exactly 25.4 millimetres. The metric system uses millimetres as a base unit for small lengths, making it precise for engineering and scientific measurements.
For example, to convert 1 inch:
- Start with 1 inch.
- Multiply 1 by 25.4.
- Result is 25.4 millimetres.
Conversion Example
- 3 inches to millimetres:
- Take 3 inches.
- Multiply 3 × 25.4 = 76.2 millimetres.
- So, 3 inches equal 76.2 millimetres.
- 0.5 inch to millimetres:
- Start with 0.5 inch.
- Multiply 0.5 × 25.4 = 12.7 millimetres.
- Therefore, 0.5 inch equals 12.7 millimetres.
- 10 inches to millimetres:
- Take 10 inches.
- Multiply 10 × 25.4 = 254 millimetres.
- Hence, 10 inches is 254 millimetres.
- 2.25 inches to millimetres:
- Start with 2.25 inches.
- Multiply 2.25 × 25.4 = 57.15 millimetres.
- Thus, 2.25 inches equal 57.15 millimetres.
Conversion Chart
| Inches | Millimetres (mm) |
|---|---|
| -24.0 | -609.6 |
| -20.0 | -508.0 |
| -15.0 | -381.0 |
| -10.0 | -254.0 |
| -5.0 | -127.0 |
| 0.0 | 0.0 |
| 1.0 | 25.4 |
| 5.0 | 127.0 |
| 10.0 | 254.0 |
| 15.0 | 381.0 |
| 20.0 | 508.0 |
| 26.0 | 660.4 |
This chart shows a selection of inch values converted into millimetres. You can find the millimetre equivalent by multiplying the inch value by 25.4. Negative values show the conversion for lengths measured in the opposite direction.
Related Conversion Questions
- How many millimetres are in exactly 1 inch?
- What is the precise millimetre value for 1 inch?
- Can 1 inch be converted to millimetres without rounding?
- Is 1 inch equal to 25 millimetres or 25.4 millimetres?
- How do I convert 1 inch to millimetres using a calculator?
- What is the formula to get millimetres from 1 inch?
- Why is 1 inch defined as 25.4 millimetres?
Conversion Definitions
Inch: An inch is a unit of length in the imperial and US customary systems, equal to 1/12 of a foot or 1/36 of a yard. Historically based on the width of a thumb, the inch is now internationally standardized as exactly 25.4 millimetres for precise measurements.
Millimetres: A millimetre is a unit of length in the metric system, equal to one thousandth of a meter (0.001 m). It is used for measuring small distances or thicknesses, widely applied in science, engineering, and everyday measurements worldwide.
Conversion FAQs
Why does 1 inch equal 25.4 millimetres?
The value 25.4 millimetres per inch was internationally agreed upon in 1959 to standardize measurements between countries using the imperial system and those using metric units. Before that, inch definitions varied slightly by region, causing confusion in engineering and trade.
Can the conversion factor change over time?
No, the conversion factor of 25.4 millimetres per inch is fixed by international agreement. Changing this would affect many industries and measurement standards globally. That constant ensures consistency across measurements and documentation worldwide.
Why do some sources show 1 inch as 25 millimetres?
Some approximate conversions round 25.4 down to 25 millimetres for simplicity in rough calculations. However, this causes inaccuracies, especially in precise fields like manufacturing or engineering where exact values matter.
How accurate is the conversion formula for practical use?
The formula multiplying inches by 25.4 is exact and suitable for any practical use. The only limitation is in the input precision; computers and calculators may round results but the formula itself does not introduce error.
Is it necessary to convert inches to millimetres in daily life?
It depends on the context. In countries using metric systems, converting to millimetres helps make sense of measurements from tools or products using inches. For example, electronics or mechanical parts often specify sizes in millimetres for standardization.
