Three answers, because "the" fraction depends on why you're asking
The exact fraction reads your decimal literally: 0.375 is 375/1000, which reduces to 3/8. That's the right answer when the decimal is exact.
Best simple approximations treat the decimal as a measurement and find the closest fraction at each denominator size, via continued fractions. This is how 3.14159 becomes the famous 22/7 (+0.040%) and the eerily good 355/113 (+0.0000085%) — fractions you'd never find by rounding denominators.
Shop fractions answer the practical question: which 16th, 32nd, or 64th of an inch is closest? 0.375 is exactly 3/8; 0.55 is 9/16 within +2.3%, or 35/64 at −0.6%.
Repeating decimals
Going the other way, a fraction's decimal form terminates only when the reduced denominator contains no prime factors other than 2 and 5. Anything else repeats forever: 9/64 = 0.140625 exactly, but 1/6 = 0.1(6…) — the digits in parentheses cycle. The calculator marks the repeating part for you.
Frequently asked questions
- Which approximation row should I use?
- The first row whose error you can live with. Each row is optimal for its denominator size — there is no better fraction with a smaller denominator than the one above it.
- Why do machinists use 64ths?
- Inch rules and drill sets are graduated in halvings — ½, ¼, ⅛, 1/16, 1/32, 1/64. A 64th is about 0.4 mm, fine enough for most bench work; beyond that, machinists switch to thousandths ("thou") and decimals.
- Can I enter a repeating decimal like 0.333…?
- Enter a few repetitions (0.333333) and look at the approximations table: 1/3 will appear immediately with a vanishing error. The exact-fraction row will show the literal 333333/1000000, which is faithful to what you typed.