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FactLab
n! = n × (n−1) × … × 1.
n! counts the number of ways to arrange n distinct items in a row — 5 books on a shelf can be ordered 5! = 120 different ways, since there are 5 choices for the first spot, 4 for the next, and so on.
Factorials explode in size — 10! is already over 3.6 million, and 20! exceeds a quintillion — which is why it's central to combinatorics, probability, and permutation/combination formulas.
n!
120
n! = n × (n−1) × … × 1
What you entered
Multiply every integer from n down to 1
5 × 4 × 3 × 2 × 1= 120Result
n!: 120
5! = 120 — the number of ways to order 5 distinct items.