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20 inches equals 508 millimeters.
To convert inches to millimeters, multiply the number of inches by 25.4, since each inch consists of exactly 25.4 millimeters, which is a fixed conversion factor.
Conversion Tool
Result in mm:
Conversion Formula
The formula to convert inches to millimeters is: millimeters = inches × 25.4. This works because one inch is exactly 25.4 millimeters by international agreement. Multiplying by 25.4 translates the length from the inch scale to the millimeter scale, which is metric.
For example, to convert 20 inches:
- Start with 20 inches.
- Multiply by 25.4: 20 × 25.4 = 508.
- The result is 508 millimeters.
Conversion Example
- Convert 12 inches to millimeters:
- 12 inches × 25.4 = 304.8 mm
- So 12 inches equals 304.8 millimeters.
- Convert 5.5 inches to millimeters:
- 5.5 × 25.4 = 139.7 mm
- Therefore, 5.5 inches equals 139.7 millimeters.
- Convert 0.75 inches to millimeters:
- 0.75 × 25.4 = 19.05 mm
- Thus, 0.75 inches equals 19.05 millimeters.
- Convert 30 inches to millimeters:
- 30 × 25.4 = 762 mm
- So, 30 inches is 762 millimeters.
Conversion Chart
| 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 |
This chart shows the equivalent millimeters for values in inches between -5.0 and 45.0. You can find the inch value in the left column, then look right to see the millimeter conversion. Negative values represent lengths in the opposite direction or depth measurements below a reference point.
Related Conversion Questions
- How many millimeters are in 20 inches exactly?
- What is the metric length for 20 inches in millimeters?
- Can 20 inches be converted to millimeters without rounding errors?
- How do I convert 20 inches to mm using a calculator?
- Is 20 inches closer to 500 or 510 millimeters?
- How to express 20 inches in millimeters for engineering drawings?
- Why does 20 inches equal 508 millimeters and not another number?
Conversion Definitions
Inches: An inch is a unit of length in the imperial system, equal to exactly 2.54 centimeters or 25.4 millimeters. It is commonly used in the United States for measuring small distances, materials dimensions, and screen sizes. The inch is subdivided into smaller parts called fractions or decimal inches.
Millimeters (mm): A millimeter is a unit of length in the metric system, equal to one thousandth of a meter. It is widely used worldwide for precise measurements in engineering, manufacturing, and everyday objects. Millimeters provide a fine scale for measuring small distances accurately, much smaller than centimeters or inches.
Conversion FAQs
Why is the conversion factor from inches to millimeters exactly 25.4?
The conversion factor 25.4 comes from an international agreement that defined the inch as exactly 25.4 millimeters in 1959. This precise value was chosen to unify measurements across countries using imperial and metric systems, eliminating previous small discrepancies.
Can I convert inches to millimeters by simply multiplying by 2.54?
No, multiplying by 2.54 converts inches to centimeters, not millimeters. Since 1 centimeter equals 10 millimeters, to get millimeters you must multiply inches by 25.4, which is 2.54 × 10.
Is the conversion tool accurate for decimal inch values?
Yes, the tool converts decimal inches accurately by applying the formula inches × 25.4. It supports fractional and decimal inputs and rounds the output to four decimal places, sufficient for most practical applications.
Can negative inch values be converted to millimeters?
Yes, negative inch values represent length or distance in the opposite direction or below a reference point. The conversion formula applies the same way, producing negative millimeter values.
Why are inches still used when millimeters are more precise?
Inches remain in use mainly in the US, UK, and some industries due to tradition, standards, and tooling. Millimeters offer finer precision and metric compatibility, but switching systems requires changes in manufacturing, regulations, and user familiarity.
