Millimeters to Inches (mm to in) Converter
Convert millimeters (mm) to inches (in) using the exact factor 1 in = 25.4 mm. Enter a value to see the decimal result and a practical fractional approximation.
How to Convert Millimeters to Inches
Use a single factor for every conversion: inches = millimeters ÷ 25.4. This page also converts the decimal into a practical fraction for tape-measure and shop use.
The Conversion Formula
The internationally accepted conversion factor is:
Therefore, to convert from millimeters to inches, you simply divide the number of millimeters by 25.4. Alternatively, you can multiply by the reciprocal, 0.0393701.
- Formula 1 (Division): Inches = Millimeters / 25.4
- Formula 2 (Multiplication): Inches = Millimeters × 0.0393701
For example, to convert 10mm to inches:
10 mm / 25.4 = 0.3937... inches
Common MM to Inches Conversion Chart
Here is a quick reference table for common millimeter to inch conversions.
| Millimeters (mm) | Inches (Decimal) | Inches (Fraction) |
|---|---|---|
| 1 mm | 0.039 in | ~ 5/128" |
| 5 mm | 0.197 in | ~ 13/64" |
| 10 mm | 0.394 in | ~ 25/64" |
| 20 mm | 0.787 in | ~ 25/32" |
| 25.4 mm | 1.000 in | 1" |
| 50 mm | 1.969 in | ~ 1 31/32" |
| 100 mm | 3.937 in | ~ 3 15/16" |
Frequently Asked Questions (FAQ)
How many mm are in one inch?
There are exactly **25.4 millimeters (mm)** in one inch (in). This definition was standardized in 1959 as part of the International Yard and Pound Agreement.
Is 10mm half an inch?
No, 10mm is not half an inch. A half-inch is 0.5 inches, which is exactly **12.7 mm**.
10mm is approximately 0.3937 inches, which is slightly more than 3/8" (0.375 in) and just under 13/32" (0.406 in).
How do you read mm to inches on a ruler?
Most standard rulers show inches on one side and centimeters (cm) / millimeters (mm) on the other. 1 centimeter is equal to 10 millimeters. To convert, you can visually compare the measurements. You will see that 2.54 cm (or 25.4 mm) lines up exactly with the 1-inch mark.
Formula (LaTeX) + variables + units
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1 inch = 25.4 millimeters
- No variables provided in audit spec.
- NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures - NIST — SI units — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/owm/metric-si/si-units
Last code update: 2026-01-19
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