Factorial Calculator - n! With Exact Digits, Trailing Zeros & Double Factorial
Factorial Calculator
Exact n! up to 10,000, with digit count, trailing zeros and n!!
Exact Results
Uses arbitrary-precision integers, so 100! and even 5000! are exact to the last digit rather than rounded to a float.
Trailing Zeros
Counts the zeros at the end of n! without computing it, by counting factors of 5.
Size at a Glance
See how many digits the answer has and its scientific notation, since factorials grow faster than almost anything.
Full Privacy
Everything is calculated in your browser.
What Is a Factorial?
The factorial n! is the product of every whole number from 1 up to n, so 5! = 5 × 4 × 3 × 2 × 1 = 120. By convention 0! = 1. Factorials count the ways to arrange n distinct items in order, which is why they sit at the heart of permutations, combinations and probability.
The double factorial n!! multiplies every second number instead: 9!! = 9 × 7 × 5 × 3 × 1 = 945 and 10!! = 10 × 8 × 6 × 4 × 2 = 3840. The number of trailing zeros in n! equals ⌊n/5⌋ + ⌊n/25⌋ + ⌊n/125⌋ + …, because each zero needs a factor of 10 and there are always more 2s than 5s.
Key Takeaways
- Big numbers: 10,000! has over 35,000 digits, and it is computed exactly.
- Full copy: Long results are abbreviated on screen; Copy gives you every digit.
- Strict input: Only whole numbers of 0 or more are accepted; anything else is reported, not guessed.