Inch fraction calculator
Add or subtract two measurements written the way a tape measure shows them, and read the answer to whatever denominator the tool in your hand actually has marks for.
= 0.375"
= 0.25"
Operation
Read to nearest
That's
5/8"
= 0.625" decimal
Worked example: 3/8" + 1/4"
3/8" plus 1/4" is 5/8", or 0.625" as a decimal, rounded to the nearest 1/16".
- 3/8" + 1/4"
- 5/8" (0.625")
- 3/8" − 1/4"
- 1/8" (0.125")
These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.
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About adding up fractions of an inch
- How do you convert a decimal inch to a fraction?
- Multiply the decimal by the marks in an inch and round to the nearest whole mark. 0.15" × 16 is 2.4, which rounds to 2 sixteenths, or 1/8" on a tape measure; read on a scale marked in thirty-seconds the same 0.15" is 5/32". The calculator does the same rounding on every answer, at whichever marks your tool has.
- What is half of 4 3/4 inches?
- 2 3/8". Halve the decimal and read it back: 4 3/4" is 4.75", and half of that is 2.375". It is the sum you do to find the centre of a board, and the one most often fumbled with the fractions left as fractions.
- How do you add fractions of an inch?
- Convert both to decimal inches, add or subtract, then convert back to the nearest mark on your tool. 3/8" is 0.375", 1/4" is 0.25", and the two together are 0.625", which is 5/8" on a tape measure. The conversion back is the step people get wrong by hand, because the decimal rarely lands exactly on a sixteenth.
- Why does this let me pick a denominator?
- A tape measure only has marks down to 1/16", but a dial caliper reads 1/64" and a good combination square's fine scale reads 1/32". Reading a caliper measurement to the nearest sixteenth throws away real precision the tool actually gave you, and reading a tape to the nearest sixty-fourth invents precision it never had.
- Why did my answer come out to 0" instead of a fraction?
- It rounded to zero at the denominator you picked — a difference of 1/64" reads as 0" at the nearest sixteenth, because a sixteenth scale has no mark that small. Switch to a finer denominator if the tool you're checking against actually has one.
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