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Millimeters ↔ Inches
Forward divides millimeters by 25.4. Reverse multiplies inches by 25.4.
Limitation: Uses the exact international inch (25.4 mm).
How to Convert mm and Inches
Pick a direction and enter a non-negative length.
Choose mm → in or in → mm.
Exact inch
1 inch = 25.4 millimeters exactly—no rounded 25.40 approximation beyond that definition.
What the Length Means
Forward divides millimeters by 25.4. Reverse multiplies inches by 25.4.
Worked Examples
25.4 mm → in
Inputs
Result
Definition of the inch.
3 in → mm
Inputs
Result
3 × 25.4 = 76.2.
Check the other direction
Flip to reverse and paste the output to confirm you land back near the original value.
Frequently Asked Questions
Still have questions about this calculation?
Try the CalculatorConversion Formulas
Documented exact or standard factors with reverse mode.
Formula
mm → in
mm ÷ 25.4
in → mm
in × 25.4
Scientific Background
Assumptions: international inch. Limitations: no fractional-inch UI (enter decimals).
Sources & review
Millimeters to Inches Calculator documents the references behind its formulas and assumptions. Always treat results as educational estimates, not personalized professional advice.
Last reviewed
August 2, 2026
Reference
nist.gov
https://www.nist.gov/pml/owm/si-units-lengthFree Calculator Hub methodology
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