1 inch is equal to 25.4 millimetres.
The inch is a unit of length commonly used in the United States and the United Kingdom. Converting inches to millimetres involves multiplying the inch value by 25.4, because one inch is defined to be exactly 25.4 millimetres. This conversion allows measurements to be represented in the metric system which is widely used internationally.
Conversion Tool
Result in millimetres:
Conversion Formula
The formula for converting inches to millimetres is:
millimetres = inches × 25.4
One inch was internationally standardized as exactly 25.4 millimetres in 1959. This means each inch measurement is multiplied by 25.4 to get the equivalent length in millimetres. The multiplication works because millimetres are smaller units, so you need more millimetres to make up the same length as an inch.
For example, converting 1 inch to millimetres step-by-step:
- Start with 1 inch
- Multiply 1 by 25.4 (1 × 25.4)
- Result is 25.4 millimetres
Conversion Example
-
Convert 2.5 inches to millimetres:
- 2.5 × 25.4 = 63.5 millimetres.
- This means 2.5 inches equals 63.5 millimetres.
-
Convert 0.75 inches to millimetres:
- 0.75 × 25.4 = 19.05 millimetres.
- The smaller fraction of an inch converts to about 19.05 millimetres.
-
Convert 10 inches to millimetres:
- 10 × 25.4 = 254 millimetres.
- This shows 10 inches is 254 millimetres long.
-
Convert 5.125 inches to millimetres:
- 5.125 × 25.4 = 130.175 millimetres.
- This number represents the millimetre equivalent of 5.125 inches.
Conversion Chart
The table below shows inches from -24.0 to 26.0 converted into millimetres. To use it, find the inch value, then read across to see the millimetre equivalent. Negative values can represent measurements in the opposite direction or below a reference point.
| Inches | Millimetres | Inches | Millimetres |
|---|---|---|---|
| -24.0 | -609.6000 | 1.0 | 25.4000 |
| -23.0 | -584.2000 | 2.0 | 50.8000 |
| -22.0 | -558.8000 | 3.0 | 76.2000 |
| -21.0 | -533.4000 | 4.0 | 101.6000 |
| -20.0 | -508.0000 | 5.0 | 127.0000 |
| -19.0 | -482.6000 | 6.0 | 152.4000 |
| -18.0 | -457.2000 | 7.0 | 177.8000 |
| -17.0 | -431.8000 | 8.0 | 203.2000 |
| -16.0 | -406.4000 | 9.0 | 228.6000 |
| -15.0 | -381.0000 | 10.0 | 254.0000 |
| -14.0 | -355.6000 | 11.0 | 279.4000 |
| -13.0 | -330.2000 | 12.0 | 304.8000 |
| -12.0 | -304.8000 | 13.0 | 330.2000 |
| -11.0 | -279.4000 | 14.0 | 355.6000 |
| -10.0 | -254.0000 | 15.0 | 381.0000 |
| -9.0 | -228.6000 | 16.0 | 406.4000 |
| -8.0 | -203.2000 | 17.0 | 431.8000 |
| -7.0 | -177.8000 | 18.0 | 457.2000 |
| -6.0 | -152.4000 | 19.0 | 482.6000 |
| -5.0 | -127.0000 | 20.0 | 508.0000 |
| -4.0 | -101.6000 | 21.0 | 533.4000 |
| -3.0 | -76.2000 | 22.0 | 558.8000 |
| -2.0 | -50.8000 | 23.0 | 584.2000 |
| -1.0 | -25.4000 | 24.0 | 609.6000 |
| 0.0 | 0.0000 | 25.0 | 635.0000 |
| 0.5 | 12.7000 | 26.0 | 660.4000 |
Related Conversion Questions
- How many millimetres are there in 1 inch exactly?
- What is the precise conversion from 1 inch to mm?
- Can I convert 1 inch to millimetres without rounding?
- Is 1 inch equal to 25 mm or 25.4 mm?
- How do I convert 1 inch to millimetres using a calculator?
- What is the formula to change 1 inch into millimetres?
- Why is 1 inch equal to 25.4 millimetres and not another number?
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. It is commonly used in the United States for measuring small distances and sizes, standardized internationally as exactly 25.4 millimetres.
Millimetres: A millimetre is a metric unit of length, equal to one thousandth of a meter. It is used worldwide for precise measurements in science, engineering, and everyday life. Millimetres are smaller than inches, allowing more detailed length expressions.
Conversion FAQs
Why is 1 inch exactly 25.4 millimetres?
The inch was internationally defined as exactly 25.4 millimetres in 1959 during an agreement among English-speaking nations. This fixed number made international trade and science easier by eliminating fractional approximations and creating a universal standard.
Can I convert inches to millimetres manually without a calculator?
Yes, by remembering that 1 inch equals 25.4 millimetres, you can multiply the inch value by 25.4 using mental math or paper. For example, 2 inches would be 2 × 25.4 = 50.8 mm. Though calculators make it easier, simple multiplications suffice for common values.
Are there any differences between US and UK inches in this conversion?
No, both US and UK inches use the same international inch defined as 25.4 millimetres since 1959. Before this agreement, small differences existed, but now the inch is standardized for all English-speaking countries for consistent measurements.
What if I need to convert fractional inches like 1/2 inch to millimetres?
First, convert the fraction to decimal (for 1/2 inch, it’s 0.5), then multiply by 25.4. So 0.5 × 25.4 = 12.7 mm. This method works for any fractional inch by changing it into a decimal before converting.
Is the conversion factor of 25.4 exact or approximate?
The conversion factor of 25.4 millimetres per inch is exact, not an approximation. This exactness ensures that measurements converted between inches and millimetres are consistent and precise worldwide, which is vital for engineering and manufacturing.