Inches to Fraction Calculator
Convert any decimal measurement to a workshop-ready fraction of an inch. Enter a decimal, a fraction, or millimetres. Pick how fine to round, and see the nearest fraction, the rounding error, and the metric equivalent.
Worked example
2.375 in to the nearest 1/16: 2.375 x 16 = 38 ticks, so 38/16 = 2 6/16 = 2 3/8 in (exact, zero error). In mm: 2.375 x 25.4 = 60.325 mm.
How the decimal-to-fraction conversion works
To convert a decimal measurement to a fractional inch, multiply the decimal by the denominator you want (8, 16, 32, 64, or 128), round the result to the nearest whole number, and place that number over the denominator. Then reduce the fraction to its lowest terms using the greatest common divisor. For example, 0.375 x 16 = 6.0, giving 6/16 which reduces to 3/8. If the measurement has a whole-inch part (say, 2.375), keep that separate and attach the fraction: 2 3/8 inch. The rounding error is simply the difference between the original decimal and the fraction expressed as a decimal. The calculator also converts your measurement to millimetres (multiply by 25.4) and centimetres (multiply by 2.54).
How the fraction-to-decimal reverse mode works
To go the other way, divide the numerator by the denominator and add the whole-inch part. For 2 3/8 inches: 3 / 8 = 0.375, then 2 + 0.375 = 2.375 in. Multiply by 25.4 to get millimetres (60.325 mm) or divide by 0.3937 to get centimetres (6.0325 cm). This is useful when a drawing or spec sheet gives dimensions as fractions and you need the decimal for a digital caliper or CNC program.
Metric to fractional inches (mm and cm input)
Enter your measurement in millimetres or centimetres using the "Input unit" dropdown. The calculator divides by 25.4 (for mm) or 2.54 (for cm) to find the decimal-inch value, then rounds it to the nearest fraction. For example, 60 mm / 25.4 = 2.3622 in, which rounds to 2 3/8 in (2.375 in) at the 1/16 level, with a rounding error of 0.0128 in. This is the standard method used in engineering drawings and conversion tables.
Choosing the right precision for your work
Most tape measures sold in the US are graduated to 1/16 inch (some to 1/32), making 1/16 the right default for general carpentry, plumbing, and DIY. Fine woodworking often calls for 1/32 inch. Machining and precision metalwork may need 1/64 inch. The 1/128 inch option is for layouts where caliper-level precision must still be expressed as a fraction, such as a very tight tolerance on a custom jig. A finer denominator always reduces the rounding error, but the fraction becomes harder to set on a tape or rule. Choose the coarsest resolution that still keeps your rounding error inside the tolerance your job allows.
Reading a tape measure
A standard US tape measure has four tiers of marks between whole-inch numbers. The longest marks land at the half-inch (1/2). The next tier lands at the quarter-inch (1/4 and 3/4). The next lands at every eighth (1/8, 3/8, 5/8, 7/8). The shortest marks land at every sixteenth. Count from the last whole inch, then add the fraction. For example, if the end of a board lines up two marks past the 2-inch mark, that is 2 2/16 = 2 1/8 inch. Converting the decimal first and then looking for the nearest fraction mark is the fastest way to transfer a caliper reading to a rule.
Common inch fractions with decimal and metric equivalents
| Fraction (in) | Decimal (in) | Millimetres (mm) | Centimetres (cm) |
|---|---|---|---|
| 1/64 | 0.0156 | 0.397 | 0.040 |
| 1/32 | 0.0313 | 0.794 | 0.079 |
| 1/16 | 0.0625 | 1.588 | 0.159 |
| 1/8 | 0.1250 | 3.175 | 0.318 |
| 3/16 | 0.1875 | 4.763 | 0.476 |
| 1/4 | 0.2500 | 6.350 | 0.635 |
| 5/16 | 0.3125 | 7.938 | 0.794 |
| 3/8 | 0.3750 | 9.525 | 0.953 |
| 7/16 | 0.4375 | 11.113 | 1.111 |
| 1/2 | 0.5000 | 12.700 | 1.270 |
| 9/16 | 0.5625 | 14.288 | 1.429 |
| 5/8 | 0.6250 | 15.875 | 1.588 |
| 11/16 | 0.6875 | 17.463 | 1.746 |
| 3/4 | 0.7500 | 19.050 | 1.905 |
| 13/16 | 0.8125 | 20.638 | 2.064 |
| 7/8 | 0.8750 | 22.225 | 2.223 |
| 15/16 | 0.9375 | 23.813 | 2.381 |
| 1 | 1.0000 | 25.400 | 2.540 |
Values listed to 4 decimal places for decimal inches and 3 decimal places for mm.
Frequently asked questions
How do I convert a decimal to a fraction of an inch?
Multiply the decimal part by the denominator you want (8, 16, 32, etc.), round to the nearest whole number, and place that over the denominator. Simplify by dividing both numbers by their greatest common divisor. For example, 0.625 x 16 = 10.0, giving 10/16 = 5/8 inch.
How do I convert millimetres to fractional inches?
Divide the millimetre value by 25.4 to get decimal inches, then round to the nearest fraction. For example, 19 mm / 25.4 = 0.7480 in, which rounds to 3/4 in (0.75 in) at the 1/16 level, with a rounding error of 0.002 in.
How do I convert a fraction back to a decimal?
Divide the numerator by the denominator, then add the whole-inch part. For 2 3/8 inch: 3 / 8 = 0.375, so 2 3/8 = 2.375 in. Multiply by 25.4 for millimetres (60.325 mm).
What denominator should I use?
Use 1/16 for general carpentry, DIY, and anything you will mark with a standard tape measure. Use 1/32 for fine woodworking or cabinet making. Use 1/64 for machining or tight-tolerance layouts. Use 1/128 only when precision equipment demands it, since marks that fine cannot realistically be set on a tape.
Why does the fraction not always match the decimal exactly?
Most decimals are not exact multiples of 1/16 (or whatever denominator you choose), so the result is rounded to the nearest tick. The rounding error shown tells you the gap. If the error exceeds the tolerance your job allows, switch to a finer denominator or use the decimal value directly in your tool.
What is a dyadic fraction and why does it matter for tape measures?
A dyadic fraction has a denominator that is a power of 2 (2, 4, 8, 16, 32, 64, 128). All standard US tape-measure graduations are dyadic, which is why this calculator only offers those denominators. A non-dyadic fraction like 1/3 or 1/5 cannot appear on a standard tape measure.