1 inch (in) is equal to 25.4 millimeters (mm).
When converting inches to millimeters, you multiply the inch value by 25.4 because one inch is exactly defined as 25.4 millimeters. This direct multiplication gives the length in millimeters.
Conversion Tool
Result in mm:
Conversion Formula
The formula to convert inches (in) to millimeters (mm) is:
mm = in × 25.4
This works because 1 inch is internationally defined as exactly 25.4 millimeters. So, multiplying the number of inches by 25.4 converts the measurement into millimeters.
Example calculation:
- If you have 3 inches, multiply 3 × 25.4 = 76.2 mm.
- If you have 0.5 inches, multiply 0.5 × 25.4 = 12.7 mm.
Conversion Example
- Convert 5 inches to mm:
- Start with 5 inches.
- Multiply 5 × 25.4 = 127.
- Result: 127 mm.
- Convert 0.75 inches to mm:
- Start with 0.75 inches.
- Multiply 0.75 × 25.4 = 19.05.
- Result: 19.05 mm.
- Convert 12.3 inches to mm:
- Start with 12.3 inches.
- Multiply 12.3 × 25.4 = 312.42.
- Result: 312.42 mm.
- Convert 0.01 inches to mm:
- Start with 0.01 inches.
- Multiply 0.01 × 25.4 = 0.254.
- Result: 0.254 mm.
Conversion Chart
| Inches (in) | Millimeters (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 |
| 6.0 | 152.4 |
| 10.0 | 254.0 |
| 14.0 | 355.6 |
| 18.0 | 457.2 |
| 22.0 | 558.8 |
| 26.0 | 660.4 |
The table shows inch values and their corresponding millimeter conversions. To use, find the inch value closest to your measurement and read across to see the millimeter equivalent. Negative values represent lengths in the opposite direction.
Related Conversion Questions
- How much is 1 inch in millimeters exactly?
- What’s the precise millimeter value of 1 in?
- Can I convert 1 inch to mm without rounding?
- How is 1 inch measured in mm for engineering tasks?
- What’s the difference between 1 inch and 25 mm?
- Is 1 inch always equal to 25.4 mm in all countries?
- How do I convert 1 in to mm using JavaScript?
Conversion Definitions
in (inch): The inch is a unit of length in the imperial and US customary systems, equal to exactly 25.4 millimeters. It’s commonly used in the United States and the United Kingdom for measuring smaller objects or distances, like paper size or screen dimensions.
mm (millimeter): The millimeter is a metric unit of length equal to one-thousandth of a meter. It’s widely used internationally in science, engineering, and everyday measurements requiring precision at a small scale, such as thickness or length of small parts.
Conversion FAQs
Why is 1 inch exactly 25.4 millimeters?
The inch was standardized internationally in 1959, fixing its length to exactly 25.4 millimeters to align measurement systems. This removes discrepancies between countries using imperial and metric standards, providing a uniform conversion rate for trade and manufacturing.
Can I use the same formula for fractions of an inch?
Yes, the formula applies to any value in inches, including fractions or decimals. Simply multiply the fractional value by 25.4 to get the equivalent millimeters. For example, 0.125 inches multiplied by 25.4 equals 3.175 mm.
Does the conversion change with temperature or material?
The conversion between inches and millimeters is fixed by definition and does not change with temperature or material. However, physical objects may expand or contract with temperature changes, but the unit conversion remains consistent regardless.
Is there a quick way to approximate 1 inch in millimeters?
A rough estimate is that 1 inch is about 25 millimeters. But for precision tasks, always use the exact value of 25.4 mm, because small differences can affect measurements in engineering or manufacturing.
Are inches and millimeters used together in technical drawings?
Some technical drawings may show dimensions in both inches and millimeters, especially when products are designed for international markets. It helps people using different measurement systems to understand sizes clearly, but it requires careful unit conversion.
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.