Quick Answer: Millimeters to Inches
One millimeter is approximately 0.03937 inches (1 mm ≈ 0.03937 in).
Conversely, 1 in = 25.4 mm.
Formula: in = mm ÷ 25.4
Worked examples
- 0.1 mm ≈ 0.003937 in
- 0.5 mm ≈ 0.01969 in
- 1 mm ≈ 0.03937 in
Method and limitations
Divide millimeters by 25.4. The international inch is exactly 2.54 cm (25.4 mm), fixed by the 1959 international yard and pound agreement. Calculations use full-precision constants; answers in this box are rounded for readability (≈ marks a rounded value) and the calculator shows up to 9 significant figures.
Each factor is exact by definition, derived from exact definitions, or a documented conventional value — see the methodology and sources.
Conversion Formula
Divide millimeters by 25.4. The international inch is exactly 2.54 cm (25.4 mm), fixed by the 1959 international yard and pound agreement.
Complete Guide to Millimeters to Inches Conversion
What is a millimeter?
The millimeter (mm) is 1/1,000 of a meter, the normal unit on engineering drawings outside the US.
What is an inch?
The inch is exactly 25.4 mm. Technical work uses decimal inches and thou (0.001 in); trade sizes often use fractions.
How the Conversion Works
in = mm ÷ 25.4. This converts units only; it does not say whether a metric and an inch part are interchangeable.
Step-by-Step Conversion Guide
Follow these steps to convert millimeters to inches manually:
Start with your value in millimeters
Example: You want to convert 1 mm
Apply the conversion formula
Use the formula: in = mm ÷ 25.4
Calculate the result
1 mm = 0.0394 in
Verify your answer
Use our calculator above to double-check your manual calculation.
Quick Mental Math Tip
Divide by 25 and take off about 1.6%: 50 mm ≈ 2 in (exact 1.969 in). 25.4 mm is exactly 1 in, 12.7 mm exactly 1/2 in, 6.35 mm exactly 1/4 in.
Common Conversions
| Millimeters (mm) | Inches (in) | Inches → Millimeters |
|---|---|---|
| 0.1 mm | 0.00393701 | 0.1 in = 2.54 mm |
| 0.5 mm | 0.019685 | 0.5 in = 12.7 mm |
| 1 mm | 0.0393701 | 1 in = 25.4 mm |
| 3 mm | 0.11811 | 3 in = 76.2 mm |
| 5 mm | 0.19685 | 5 in = 127 mm |
| 6.35 mm | 0.25 | 6.35 in = 161.29 mm |
| 10 mm | 0.393701 | 10 in = 254 mm |
| 12.7 mm | 0.5 | 12.7 in = 322.58 mm |
| 25.4 mm | 1 | 25.4 in = 645.16 mm |
| 100 mm | 3.93701 | 100 in = 2,540 mm |
Popular Millimeters to Inches Conversions
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When You'll Need This Conversion
Material thickness
Sheet, plate and board thickness specified in mm against inch-based stock or gauges.
Tolerances and drawings
Read a metric dimension or tolerance in decimal inches or thou.
Pro Tips for Accurate Conversions
Convert tolerances separately
±0.05 mm is ±0.00197 in. Convert the tolerance as well as the nominal size, and keep the precision the drawing actually specifies.
Common Mistakes to Avoid
Confusing mm and cm
A factor of 10 error.
100 mm = 3.937 in; 100 cm = 39.37 in.
Assuming a "close" fractional size is equivalent
Nearest-fraction rounding can be off by more than a typical fit tolerance.
Use the decimal-inch result, then decide on the fraction or size against the actual requirement.
Frequently Asked Questions
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Conversion factors on this page are either exact by definition, derived from exact definitions, or conventional values in documented use. Each is listed with its source in our conversion methodology and sources, which cites the BIPM SI Brochure, NIST Special Publication 811 and the relevant IEC or national standards. Results are rounded for display only; calculations use full precision.