How the three forms relate
A percentage is a number of hundredths, so 37.5% is 37.5 ÷ 100 = 0.375. A decimal becomes a fraction by writing its digits over the matching power of ten — 0.375 is 375/1000 — and dividing the top and bottom by their greatest common divisor, here 125, which leaves 3/8. Going the other way, a fraction becomes a decimal by dividing the top number by the bottom one.
This converter does all of that with exact whole-number arithmetic rather than floating point, so 0.1 stays 1/10 and never turns into 0.1000000000000000055, which is what a computer's ordinary number type actually stores.
Repeating decimals
A fraction in lowest terms has a decimal that ends only when its bottom number has no prime factors other than 2 and 5. Every other fraction repeats forever: 1/3 is 0.333…, 1/6 is 0.1666…, 1/7 is 0.142857142857…. The result shows the repeating block in brackets — 0.(3), 0.1(6), 0.(142857) — and a rounded version beside it for copying into something that needs a plain number.
You can type repeating decimals the same way. 0.(3) is read as exactly one third. A value ending in an ellipsis, such as 0.1666…, is read as repeating when its last digit or block of up to six digits appears twice in a row (or when only one digit follows the point); otherwise the converter takes the simplest fraction within one unit of the last digit you typed, which is how 0.6667… becomes 2/3.
Percentages over 100 and below zero
Nothing stops a percentage going past 100: 150% is 1.5, or 3/2, or the mixed number 1 1/2. Negative values convert the same way with the sign kept, so -25% is -0.25 and -1/4. To work out a percentage of an amount rather than convert between forms, use the percentage calculator.