Convert Inches Millimeters Calculator
Convert inches to millimeters calculator works out Converted value and Unrounded result from Value, Reading and Ruler divisions per inch. The default figures on this page give converted value of 25.4; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
2 steps
- 1 in = 25.4 mm
- 1 x 25.4 = 25.4 mm
Its 12 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
2 sources
What the convert inches to millimeters calculator calculates
Converts a quantity between any two units of the same physical family using exact SI-derived factors.
How to use this convert inches to millimeters calculator
Enter Value, Reading, Ruler divisions per inch and Feet. Converted value updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Converted value, Unrounded result and Conversion.
The method follows NIST Special Publication 811, Guide for the Use of the International System of Units and BIPM, The International System of Units (SI Brochure).
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do you convert inches to mm?
Converts a quantity between any two units of the same physical family using exact SI-derived factors. Open “How this is worked out” beside the result to see each step for your own figures.
What is 5 inches in millimeters?
Converts a quantity between any two units of the same physical family using exact SI-derived factors. On this page it is computed from Value, Reading and Ruler divisions per inch and shown as Converted value, Unrounded result and Conversion.
What information do I need for the convert inches to millimeters calculator?
You need Value, Reading, Ruler divisions per inch, Feet, Inches and Significant figures. The page opens with default figures that give converted value of 25.4, so you can see the scale before entering your own.
What does this convert inches to millimeters calculator show?
It shows Converted value, Unrounded result, Conversion, Reading, Multiply by and Reverse conversion, with the working and any warnings for your entries.
Can I check how the convert inches to millimeters calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.