About the Inch Fraction Converter
This inch fraction converter rounds a decimal inch measurement — like 1.37 in from a set of calipers or a CAD drawing — to the nearest fraction you can actually find on a tape measure or ruler, such as 1 3/8". Choose the precision (halves down to 64ths) and the tool reduces the fraction to lowest terms, shows the decimal value of that fraction, the rounding error, and the measurement in millimeters.
It also works the other way: switch the mode and type a fraction such as 2 5/16 or 7/8 to get its exact decimal and metric equivalent. Woodworkers, machinists, 3D-printing hobbyists, DIYers reading imperial plans and anyone matching drill bits or sockets to a measurement will find it useful. The table lists every step at your chosen precision with decimal and mm values.
Rounding goes to the nearest fraction (halves round up), and the error line tells you how far the fraction is from the true value, so you can decide whether 1/16" precision is good enough or you need 1/32" or 1/64". One inch is exactly 25.4 mm.
How to use the inch fraction converter
- 1Choose whether you are converting a decimal to a fraction or a fraction to a decimal.
- 2Enter the measurement (for fractions, type it like 1 3/8 or 7/8).
- 3Pick the precision — 1/16" suits tape measures, 1/32" or 1/64" suit machining.
- 4Read the fraction, its decimal value, the rounding error and the millimeter equivalent.
Formula and method
To find the nearest fraction, the decimal is multiplied by the chosen denominator D (for example 16), rounded to the nearest whole number n, and written as n/D. Whole inches are pulled out to make a mixed number, and the remaining fraction is divided by the greatest common divisor so 6/16 becomes 3/8.
Going from a fraction to a decimal simply divides the numerator by the denominator and adds the whole inches. The rounding error is the fraction’s value minus the exact value, and millimeters multiply the exact inches by 25.4.
- D
- Denominator / precision: 2, 4, 8, 16, 32 or 64
- n
- Number of 1/D steps, rounded to the nearest whole number
Worked examples
1.37 inches to the nearest 1/16
1.37 × 16 = 21.92, which rounds to 22 sixteenths. 22/16 = 1 6/16 = 1 3/8" (1.375 in), just 0.005" above the true value.
0.8125 inches
0.8125 × 16 = 13 exactly, so the measurement is 13/16" with no rounding error. In metric it is 0.8125 × 25.4 = 20.6375 mm.
2.53 inches to the nearest 1/32
2.53 × 32 = 80.96, which rounds to 81. 81/32 = 2 17/32" = 2.53125 in, an error of about 0.001".
Fraction to decimal: 1 3/8"
3 ÷ 8 = 0.375, plus the whole inch gives 1.375 in. Multiplying by 25.4 gives 34.925 mm.
Frequently asked questions
How do I convert a decimal to a fraction of an inch?+
Multiply the decimal part by 16 (or 32, 64), round to a whole number and put it over 16. For 0.3 in: 0.3 × 16 = 4.8 → 5, so it is about 5/16". Then reduce the fraction if possible.
What is 0.75 inches as a fraction?+
0.75 inches is exactly 3/4". Other common ones are 0.25 = 1/4", 0.5 = 1/2", 0.125 = 1/8", 0.375 = 3/8" and 0.625 = 5/8".
What precision should I use?+
Most tape measures are marked in 1/16", which suits carpentry and DIY. Fine woodworking often uses 1/32", while machinists and gunsmiths may need 1/64" or decimal thousandths.
How many millimeters is 1/16 of an inch?+
1/16" equals 1.5875 mm, so each sixteenth mark on a tape measure is just over 1.5 mm apart. 1/32" is 0.794 mm and 1/64" is 0.397 mm.
How do I read fractions on a tape measure?+
The longest marks are inches, then halves, quarters, eighths and the shortest are sixteenths. Count sixteenths past the last whole inch, then reduce: 12 sixteenths past 3" is 3 12/16 = 3 3/4".