Factorial Calculator
Calculate n! or the double factorial n!! exactly for any whole number up to 5000, with digit count, trailing zeros and scientific notation.
Input
Whole number from 0 to 5000
Output
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/factorial-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"n": "10",
"kind": "factorial"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `factorial-calculator` tool (Factorial Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
Embed widget
<iframe
src="https://iotools.cloud/embed/factorial-calculator/"
width="100%" height="520" frameborder="0" scrolling="no" loading="lazy"
title="Factorial Calculator — iotools.cloud"
sandbox="allow-scripts allow-forms allow-same-origin allow-downloads allow-popups allow-popups-to-escape-sandbox"
allow="clipboard-write"
style="width:100%;border:1px solid #e5e7eb;border-radius:12px;overflow:hidden"></iframe>
<script src="https://iotools.cloud/embed.js" async></script>Drop this into your own page — free, no key required, just a link back.
| Cost per API/MCP call | From 5 credits |
|---|---|
| Need more credits? | View pricing |
Also available with
Guides
What is a factorial?
The factorial of a non-negative integer n, written n!, is the product of every whole number from 1 up to n. For example, 5! = 5 × 4 × 3 × 2 × 1 = 120. By convention 0! = 1. Factorials count the ways to arrange n distinct items and appear throughout permutations, combinations, probability and series expansions.
How this tool works
- Enter a whole number from 0 to 5000.
- Choose Factorial (n!) or Double factorial (n!!).
- Read the exact result, its scientific notation, the number of digits and the number of trailing zeros.
All arithmetic uses arbitrary-precision integers, so results are exact rather than rounded floating-point approximations. Very long results are shortened on screen, but Copy and Download give you the full value.
Double factorial
The double factorial n!! multiplies every second number down from n: 9!! = 9 × 7 × 5 × 3 × 1 = 945. It is not the factorial of a factorial.
Trailing zeros
Each trailing zero in n! comes from a factor pair of 2 and 5. Since 2s are plentiful, the count equals ⌊n/5⌋ + ⌊n/25⌋ + ⌊n/125⌋ + …; 100! ends with 24 zeros.
Privacy
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