The factorial of n, written n!, is the product of all whole numbers from 1 to n. So 5! = 5 × 4 × 3 × 2 × 1 = 120. Factorials count arrangements: 10 people can stand in a line in 10! = 3,628,800 different orders. By convention 0! = 1, which keeps permutation and combination formulas consistent. Factorials grow extremely fast. 20! is already over 2.4 quintillion, and 21! exceeds what standard 64-bit integers can hold. This calculator uses big-number arithmetic and accepts whole numbers from 0 through 170. It is useful for probability, combinatorics, statistics and programming practice.
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