What is the Factorial Calculator?
A factorial multiplies a whole number by every positive whole number below it. Factorials count the ways to arrange items and appear throughout probability and combinatorics.
How to use the Factorial tool
- Enter Whole number.
- The result updates instantly as you type, or press Calculate.
- Use Try an example to see how it works, and Reset to start again.
Formula
n! = n × (n − 1) × (n − 2) × … × 2 × 1, and 0! = 1.
Worked example
With whole number 10, the 10! comes to 3,628,800. Number of digits: 7.
Frequently asked questions
Why is 0! equal to 1?
There is exactly one way to arrange zero items, so by convention 0! = 1.
How big can factorials get?
Very fast. 20! is over 2 quintillion and 100! has 158 digits.
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Checked by Calcovate · Formula tested against worked examples · Updated 7 Nov 2026 · How we check our calculators.