Convert mm to inches

Length Converter with Millimeter (mm) to Inch (in) Conversion

Converting millimeters (mm) to inches (in) is essential for reading metric dimensions on engineering drawings, 3D printer specs, jewelry sizes, and imported hardware when you think in imperial units. One inch is defined as exactly 25.4 millimeters, so inches = millimeters ÷ 25.4. That makes 25.4 mm equal to exactly one inch.

Millimeter to Inch Conversion Formula

in = mm ÷ 25.4

Divide the length in millimeters by 25.4 to convert to inches. The factor 25.4 is an exact definition tied to the international inch, not a rounded approximation.

Common millimeter to inch values

Sample Calculation: Bolt or Bearing Size

A datasheet lists a shaft diameter of 10 mm. What is that in inches?

• Start with the measurement in millimeters: 10 mm
• Divide by the conversion factor: 10 ÷ 25.4
• Perform the division: 0.393700787…

10 mm is approximately 0.394 inches—often written as 0.39 in for rough fits.

About the Units

One Millimeter (mm) equals 0.03937 Inch (in)

One Inch (in) equals 25.4 Millimeters (mm)

Where This Conversion Is Used

• Convert Millimeter to Inch to ensure consistent values across calculations, reports, and measurement systems.
• Helps compare dimensions when switching between metric and imperial measurements, commonly required when reading construction drawings, machining tolerances, and US-based product specifications.
• Frequently used to validate dimensions taken from CAD models, blueprints, survey maps, and engineering documents that mix inch- and meter-based units.

Frequently Asked Questions

References

• NIST: Metric System (SI)