Your mm ⇄ Inch Fraction & Tolerance Converter
Convert millimetres to inches as decimals and the nearest fraction (1/2 to 1/128), or fractions back to mm, with tolerance-aware min/max limits.
- Free to use
- No signup
- Nothing to install
- Runs in your browser
Illustration. Open the tool for the real thing.
See it in action
The mm ⇄ Inch Fraction & Tolerance Converter with sample data entered and the result showing.
What is the mm ⇄ Inch Fraction & Tolerance Converter?
Machinists, woodworkers and CAD users keep running into the same problem: a dimension in millimetres needs to become a fraction on a tape measure, or a fractional drawing needs real millimetres. This converter accepts decimals or fractions (12.7, 3 7/16, 1-1/4, 5/32, 0.5in, 3/8") in millimetres, inches, centimetres, metres or thou, and returns the value in every unit plus the nearest fraction at the precision you choose (halves to 1/128) — together with how far that fraction is from the exact value.
Because a fraction is only an approximation, the tool is tolerance-aware: enter a symmetric or asymmetric tolerance and it computes the minimum and maximum limits in millimetres, decimal inches and fractions, and warns when the rounding error of the fraction exceeds your tolerance. Batch mode converts a whole list of values at once and copies it as CSV, and a 1/64 fraction chart gives the decimal and mm for every step up to 1".
3 steps to your result
Open the tool and follow along. Nothing to install.
What you get
Decimals, fractions and mixed numbers, with or without units.
1/2 to 1/128 with the exact rounding error in mm.
Min/max in mm, inches and fractions; warns when rounding exceeds tolerance.
List conversion with CSV copy and a 1/64 chart.
AZRS tools are free to open and use.
It opens in your browser, so there is nothing to download.
Used for
Pick one to see how it fits.
How it compares
A plain look at this tool next to the usual alternative.
| This engineering calculator | Desktop software | |
|---|---|---|
| Install | Nothing to install | Usually an app or program to install |
| Account | No signup | Depends on the product |
| Cost | Free to use | Free or paid, depends on the product |
| Where it runs | In your browser tab | On your computer |
Questions people ask
How do I convert mm to a fraction?
Divide by 25.4 to get inches, then round to the nearest 1/n; this tool does it and shows the error.
Can it convert fractions to mm?
Yes — type the fraction (3/8") and choose any output.
What fraction precision should I use?
1/16 for woodworking, 1/64 for machining, decimals for precision parts.
Is batch conversion supported?
Yes, one value per line with CSV copy.
How do I use the mm ⇄ Inch Fraction & Tolerance Converter?
1. Type a value (e.g. 12.7 mm or 3 7/16"). 2. Choose the unit and fraction precision. 3. Add a tolerance if the part has one. 4. Copy the result or use Batch mode for many values.
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Try the mm ⇄ Inch Fraction & Tolerance Converter now
Millimetres to inch fractions with tolerance limits and a batch list. Free to use.
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