1 inch equals 25.4 millimetres.
The conversion from inches to millimetres is based on a fixed ratio where one inch is equivalent to exactly 25.4 millimetres. This is a standard measurement used internationally for precision in length conversions.
Conversion Tool
Result in millimetres:
Conversion Formula
The formula to convert inches to millimetres is:
millimetres = inches × 25.4.
This works because one inch is defined as exactly 25.4 millimetres internationally. So, when you multiply the number of inches by 25.4, you get the equivalent length in millimetres. The number 25.4 is a fixed constant, no matter the measurement context.
Example calculation for 1 inch:
1 inch × 25.4 = 25.4 millimetres
Conversion Example
- Convert 3 inches to millimetres:
- Multiply 3 by 25.4
- 3 × 25.4 = 76.2 millimetres
- The result is 76.2 millimetres
- Convert 0.5 inches to millimetres:
- Multiply 0.5 by 25.4
- 0.5 × 25.4 = 12.7 millimetres
- The result is 12.7 millimetres
- Convert 7.25 inches to millimetres:
- Multiply 7.25 by 25.4
- 7.25 × 25.4 = 184.15 millimetres
- The final value is 184.15 millimetres
Conversion Chart
This table shows inch values from -24.0 to 26.0 converted into millimetres. You can find the millimetres by multiplying the inch value by 25.4. Negative values represent lengths in the opposite direction or below zero.
| Inches | Millimetres | Inches | Millimetres |
|---|---|---|---|
| -24.0 | -609.6 | 2.0 | 50.8 |
| -20.0 | -508.0 | 6.0 | 152.4 |
| -16.0 | -406.4 | 10.0 | 254.0 |
| -12.0 | -304.8 | 14.0 | 355.6 |
| -8.0 | -203.2 | 18.0 | 457.2 |
| -4.0 | -101.6 | 22.0 | 558.8 |
| 0.0 | 0.0 | 26.0 | 660.4 |
| -23.0 | -584.2 | 1.0 | 25.4 |
| -19.0 | -482.6 | 5.0 | 127.0 |
| -15.0 | -381.0 | 9.0 | 228.6 |
| -11.0 | -279.4 | 13.0 | 330.2 |
| -7.0 | -177.8 | 17.0 | 431.8 |
| -3.0 | -76.2 | 21.0 | 533.4 |
| 3.0 | 76.2 | 25.0 | 635.0 |
| 4.0 | 101.6 | 0.5 | 12.7 |
Related Conversion Questions
- What is the exact length of 1 inch in millimetres?
- How many millimetres are in one inch for precise measurements?
- Can 1 inch be converted to millimetres without rounding?
- Is 1 inch equal to 25.4 millimetres in all measuring systems?
- What formula should I use to convert 1 inch to millimetres?
- How do I convert 1 inch to millimetres using a calculator?
- Why is 1 inch defined as 25.4 millimetres exactly?
Conversion Definitions
Inch: An inch is a unit of length used mainly in the United States and UK, equal to 1/12th of a foot or 2.54 centimetres. It is traditionally used in measuring small lengths and dimensions in everyday objects, manufacturing, and engineering.
Millimetres: Millimetres are a metric unit of length in the International System, equal to one thousandth of a metre. It is used worldwide for precise, small-scale measurements in science, engineering, and construction.
Conversion FAQs
Why is one inch exactly 25.4 millimetres?
The inch was internationally defined as exactly 25.4 millimetres in 1959 by an agreement between English-speaking countries. This was to standardize measurements, avoiding differences between earlier definitions based on physical objects or other standards.
Can the conversion from inches to millimetres cause errors?
Conversion itself is precise, since 1 inch equals exactly 25.4 millimetres. However, errors may appear if rounding occurs or measuring tools are inaccurate. Using decimal places helps reduce these errors when converting values.
Is the conversion factor always 25.4 for all types of inches?
Yes, the international inch is defined as 25.4 mm. Though historical inches varied slightly by country or industry, modern measurements use this fixed value for consistency globally.
How does temperature affect inch to millimetre conversion?
Temperature can cause materials to expand or contract, affecting physical measurements. But the conversion factor between inch and millimetres remains constant since it defines units, not actual material size changes.
How to convert fractional inches to millimetres?
First, convert the fraction to a decimal, then multiply by 25.4. For example, 1 1/2 inches equals 1.5 inches; multiplying 1.5 × 25.4 gives 38.1 millimetres.
Last Updated : 05 July, 2025
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Sandeep Bhandari holds a Bachelor of Engineering in Computers from Thapar University (2006). He has 20 years of experience in the technology field. He has a keen interest in various technical fields, including database systems, computer networks, and programming. You can read more about him on his bio page.