About the Factorial Calculator
The factorial of a whole number n, written n!, is the product of all whole numbers from 1 up to n. So 5! is 5 × 4 × 3 × 2 × 1 = 120, and by definition 0! is 1. Factorials grow very fast: 20! already has 19 digits and 100! has 158. This calculator uses exact big-number arithmetic, so you get every digit rather than a rounded decimal. It also shows the number of digits, the number of trailing zeros, a scientific-notation form and the double factorial n!!. It handles whole numbers from 0 to 3,000.
Updated October 2026
How to use the Factorial Calculator
- 1Type a whole number from 0 to 3,000 in the number box.
- 2Read n! in the highlighted answer; long results switch to scientific form.
- 3Open the full digits box underneath to see or copy every digit.
- 4Check the extra rows for trailing zeros, digit count and the double factorial.
- 5Press Copy result to copy the exact value.
What factorials are used for
Factorials count ordered arrangements. Five people can stand in a queue in 5! = 120 different orders. They appear in probability, in the binomial theorem, in Taylor series for functions such as sine and the exponential, and in algorithms that try every ordering of a list. Because they grow faster than any power, even small inputs give huge answers, which is why exact digits are useful.
How fast do factorials grow?
- 10! = 3,628,800
- 15! = 1,307,674,368,000
- 20! = 2,432,902,008,176,640,000
- 50! has 65 digits
- 100! has 158 digits and ends with 24 zeros
Reading very long results
For results longer than about 24 digits the headline shows scientific notation, for example 9.332621544e157 for 100!, meaning 9.33 followed by 157 more digits. The exact digits are in the box below the answer. The digit count tells you the size of the number at a glance, and the trailing zeros show how many times the result divides by ten.
Related maths
Permutations and combinations are built from factorials, so the Permutation and Combination Calculator is the natural next step. The Probability Calculator and Exponent Calculator are useful companions, and the Scientific Notation Converter helps you read huge answers. If you want the Fibonacci sequence instead, try the Fibonacci Calculator.
Features
- Exact results using big-number arithmetic
- Works for every whole number from 0 to 3,000
- Shows the number of digits
- Counts trailing zeros
- Scientific notation for very large results
- Double factorial (n!!)
- Copy the full value
- Runs in your browser
Frequently asked questions
What is a factorial?
A factorial multiplies a whole number by every positive whole number below it. 6! equals 6 × 5 × 4 × 3 × 2 × 1, which is 720. It is written with an exclamation mark and is used to count the ways of arranging things in order.
Why is 0! equal to 1?
There is exactly one way to arrange zero items: do nothing. Defining 0! as 1 also keeps formulas for permutations and combinations working, and it follows the pattern n! = n × (n−1)!, since 1! = 1 × 0! only works if 0! is 1.
How do I calculate the factorial of a large number?
Multiplying by hand quickly becomes impractical, and normal calculators switch to rounded scientific notation after about 20!. This page uses big-number arithmetic, so 100! shows all 158 digits exactly, and you can copy them.
How many trailing zeros does a factorial have?
Each trailing zero comes from a factor of 10, which is a pair of 2 and 5. There are always more 2s than 5s, so the count equals the number of factors of 5: divide n by 5, by 25, by 125 and so on, and add the whole parts. 100! ends in 24 zeros.
What is a double factorial?
The double factorial n!! multiplies every second number going down from n. So 7!! is 7 × 5 × 3 × 1 = 105 and 8!! is 8 × 6 × 4 × 2 = 384. It is not the same as taking the factorial twice.
Can I find the factorial of a decimal or a negative number?
Not with this calculator. Ordinary factorials are defined only for whole numbers from 0 upwards. Extensions to decimals use the gamma function, and negative whole numbers have no finite factorial.