Factorial Calculator

The factorial of a whole number n, written n!, is the product of every whole number from n down to 1. For example, 5! = 5 × 4 × 3 × 2 × 1 = 120. It counts the number of ways to arrange n distinct items in order.

0! equals 1. There is exactly one way to arrange zero items (the empty arrangement), and defining 0! = 1 keeps formulas like permutations and combinations consistent. This calculator returns 1 for 0!.

No. The ordinary factorial is only defined for whole numbers of 0 or greater. If you enter a negative number or a decimal, the calculator tells you the result is undefined instead of showing a wrong value. (The gamma function extends factorials to other numbers, but this tool computes the standard whole-number factorial.)

Factorials grow extremely fast — 100! already has 158 digits. The calculator computes the exact value using arbitrary-precision integers, shows the digit count, and displays a short scientific-notation form (like 9.3326 × 10^157) so huge results stay readable, while the full exact digits are still available.