Factorial calculator

Type a whole number up to 5,000. The factorial appears in full as you type, with its number of digits and how many zeros it ends in.

The answers are exact, not rounded. A phone calculator shows 25! as about 1.55 × 10^25; this gives all 26 digits.

n! means every whole number from n down to 1 multiplied together. 5! is 5 × 4 × 3 × 2 × 1 = 120, and it counts the ways to put 5 things in a row.

The numbers grow fast. 10! is already 3,628,800, and 52!, the number of ways to shuffle a deck of cards, is about 8.07 × 10^67.

0! is 1. There's exactly one way to arrange nothing, and the rule n! = n × (n − 1)! needs it: 1! = 1 × 0!.

Every trailing zero comes from a 10, which is a 2 times a 5. There are always more 2s than 5s among the factors, so counting the 5s gives the zeros. Divide n by 5, then by 25, then by 125, rounding each down, and add them up. For 100! that's 20 + 4 = 24 zeros.

The French mathematician Christian Kramp introduced the n! sign in 1808. The idea is much older. A Jain text, the Anuyogadvāra-sūtra, is one of the earliest known to describe factorials, and in 1677 Fabian Stedman used them to count the orders in which a set of church bells can ring.