20 inches is equal to 508 millimeters.
To convert inches to millimeters, you multiply the length value by 25.4, since one inch equals exactly 25.4 millimeters. So, multiplying 20 by 25.4 gives the result in millimeters.
Conversion Tool
Result in mm:
Conversion Formula
The conversion from inches to millimeters uses the formula:
millimeters = inches × 25.4
This works because 1 inch is defined exactly as 25.4 millimeters by international agreement. To convert any length from inches to mm, you just multiply the number of inches by 25.4.
Example: converting 20 inches
- Start with 20 inches
- Multiply 20 by 25.4
- 20 × 25.4 = 508 mm
So, 20 inches equals 508 millimeters.
Conversion Example
- 15 inches to mm:
- Take 15 inches
- Multiply 15 × 25.4
- Result: 381 mm
- 7.5 inches to mm:
- Start with 7.5 inches
- 7.5 × 25.4 = 190.5
- So, 7.5 inches is 190.5 mm
- 3 inches to mm:
- 3 × 25.4 = 76.2
- This gives 76.2 millimeters
- 12.25 inches to mm:
- Multiply 12.25 by 25.4
- 12.25 × 25.4 = 311.15 mm
Conversion Chart
The chart below shows inches values from -5.0 to 45.0 and their mm equivalents. You can find any value close to yours and see the mm conversion without calculation.
| Inches | Millimeters (mm) |
|---|---|
| -5.0 | -127.0 |
| -2.5 | -63.5 |
| 0.0 | 0.0 |
| 2.5 | 63.5 |
| 5.0 | 127.0 |
| 7.5 | 190.5 |
| 10.0 | 254.0 |
| 12.5 | 317.5 |
| 15.0 | 381.0 |
| 17.5 | 444.5 |
| 20.0 | 508.0 |
| 22.5 | 571.5 |
| 25.0 | 635.0 |
| 27.5 | 698.5 |
| 30.0 | 762.0 |
| 32.5 | 825.5 |
| 35.0 | 889.0 |
| 37.5 | 952.5 |
| 40.0 | 1016.0 |
| 42.5 | 1079.5 |
| 45.0 | 1143.0 |
Related Conversion Questions
- How many millimeters are in 20 inches exactly?
- What is the mm equivalent of a 20 inch object?
- Convert 20 inches to millimeters with decimal precision?
- Is 20 inches equal to 500 mm or more?
- How do I change 20″ into millimeters quickly?
- What’s the difference between 20 inches and 20 mm in size?
- Can I get the conversion factor used for 20 inches to mm?
Conversion Definitions
Inches: An inch is a unit of length in the imperial and US customary systems, defined as exactly 25.4 millimeters. It is commonly used in the United States, Canada, and UK for measuring smaller lengths, such as in construction, engineering, and everyday items.
Millimeters (mm): A millimeter is a metric unit of length equal to one-thousandth of a meter. It is widely used around the world for precise measurements in science, engineering, and manufacturing, representing very small distances or thicknesses.
Conversion FAQs
Why is one inch equal to exactly 25.4 millimeters?
Since 1959, the inch was internationally defined as exactly 25.4 millimeters to create a standard between imperial and metric systems. This agreement allows consistent conversions without rounding errors and is used globally in measurements and manufacturing.
Can I convert inches to mm without a calculator?
Yes, you can estimate by multiplying inches by 25 and adding a bit more. For example, for 20 inches, 20 × 25 = 500, then add 20 × 0.4 = 8, total 508 mm. But for precise work, using the exact factor 25.4 is recommended for accuracy.
Are millimeters used everywhere instead of inches?
No, millimeters are part of the metric system and used mostly outside the US. Inches are still common in the US, UK, and Canada, especially in construction and tools. Metric units are preferred in science and worldwide industries for standardization.
What happens if I convert negative inches to millimeters?
Negative inches represent lengths in the opposite direction or below a reference point. When converted, the negative sign remains, so -5 inches equals -127 millimeters. The conversion formula stays the same, just with a negative input.
Is the inch to millimeter conversion exact or approximate?
The conversion is exact. One inch is defined as exactly 25.4 millimeters internationally, so no approximations or rounding errors occur in calculations based on this definition.
Last Updated : 04 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.