Enter a non-negative whole number to calculate its factorial (n!) with exact precision.
How to Use the Factorial Calculator
One field, a whole number, and the exact factorial comes back — no rounding, even for values large enough that a typical calculator would switch to scientific notation.
5! is 5 × 4 × 3 × 2 × 1, or 120 — every whole number counting down to 1, multiplied together. This Factorial Calculator computes n! exactly, using full-precision integer math so results don't blur into rounding error once they get large, which they do, fast.
10! is already 3,628,800. 20! clears 2.4 quintillion. Each additional number multiplies the whole running product rather than adding to it, so factorials outrun even exponential growth once n gets past a certain point — this is the steepest-growing common function you'll run into in ordinary math.
0! being defined as 1, not 0, looks like an arbitrary rule until you notice why: choosing zero items from a set has exactly one way to do it (choose nothing), and combinatorics formulas need 0! = 1 to stay consistent with that. Which is really what factorials are for — counting how many ways a set of things can be arranged or chosen, the foundation under permutations, combinations, and a good chunk of probability theory.