What is factorial? How to calculate factorial of a number?

Definition of factorial

The factorial is the product of an integer and all the integers below it; e.g. factorial four ( 4! ) is equal to 24 (4 x 3 x 2 x 1). The factorial value of 0 is by definition equal to 1. For negative integers, factorials are not defined. The factorial can be seen as the result of multiplying a sequence of descending natural numbers (such as 3 x 2 x 1). The factorial symbol is the exclamation mark (!).

The factorial formula

If n is an natural number greater than or equal to 1, then n! = n x (n - 1) x (n - 2) x (n - 3) ... 3 x 2 x 1

If p = 0, then p! = 1 by convention.

Example: 6! = 6 x 5 x 4 x 3 x 2 x 1 = 720

Table of Factorials