Inch ⇄ Millimeter (mm) Converter

Inches ⇄ Millimeters – Exact 25.4 Factor Guide

Core idea: The inch is defined so that 1 inch = exactly 25.4 millimeters (since 1959 international agreement). Converting is pure multiplication or division—no approximation needed.

Formulas

• mm = in × 25.4
• in = mm ÷ 25.4
• Through centimeters: mm = (in × 2.54) × 10
• Fractions: (a/b) in → mm = (a × 25.4) ÷ b
• Reverse fraction (approx): convert decimal inches to nearest common fraction (1/16, 1/32, etc.).

Quick Values

Worked Example

Example: Convert 2 3/8 in to mm.
Fraction → decimal: 3/8 = 0.375 → 2.375 in.
mm = 2.375 × 25.4 = 60.325 mm (exact arithmetic, rounded display).
Reverse check: 60.325 ÷ 25.4 = 2.375 in.

Where Used

• Manufacturing drawings (metric specs vs imperial hardware).
• 3D printing & CNC (tool libraries metric; models often in inches).
• DIY woodworking (fractional tapes → metric machine settings).

Accuracy & Tips

• Use the exact 25.4; never round to 25 or 25.4± shortened—small errors amplify in assemblies.
• For fractions: convert numerator/denominator to decimal before multiplying.
• When converting mm to an inch fraction: divide by 25.4, then multiply remainder by 16 or 32 and round to nearest.

More Examples

FAQ

• Why is 25.4 exact? Defined by fixing the yard at 0.9144 m; inch = yard ÷ 36.
• How to get a fraction from mm? mm ÷ 25.4 → decimal inches → multiply decimal part by 16 (or 32) and round.
• Largest common error? Using 0.039 or 25 instead of exact factors—produces noticeable drift in stacked parts.

All conversions use the exact 25.4 factor; only final outputs rounded for readability.