How to Convert mm to Fractional Inches

Written by paul dohrman
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How to Convert mm to Fractional Inches
There are 25.4 millimetres in an inch. (measuring tool image by Michalis Palis from

Converting millimetres (mm) to fractional inches is a matter of rounding to the nearest 16th of an inch, since this is how far inches are broken down on rulers. The conversion factor between inches and mm is 25.4.

Skill level:


  1. 1

    Measure or identify the length in mm for the object of interest.

  2. 2

    Divide the amount in Step 1 by 25.4 to get the inches in decimal form.

  3. 3

    Take the decimal portion of the result of Step 2 and multiply it by 16. The point is to determine the nearest 16th of an inch on a ruler.

    For example, 120mm divided by 25.4 yields 4.7244 inches. Multiplying the 0.7244 by 16 gives 11.591. Round this to 12/16, or 3/4. The result, therefore, is 4 3/4 inches.

Tips and warnings

  • If you’re making the conversion from mm to fractional inches for tools, you’ll probably want to use 32 instead of 16. Following the exact same calculations but using 32 instead of 16, the answer for the example would become 4 23/32 inches. Wire width conversion could entail converting to the nearest 64th of an inch.

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