Prime Number Checker
Check whether a number is prime, and factorise it if it is not.
Your result will appear here as soon as you add some input.
How to use it
- 1Enter your values in the fields above.
- 2The answer and the working appear immediately below.
- 3Check the working to see the formula with your numbers substituted.
About the Prime Number Checker
A prime has exactly two distinct divisors: one and itself. That definition is why 1 is not prime - it has only one divisor - and the reason mathematicians care is that it keeps the fundamental theorem of arithmetic true.
Every whole number above 1 has exactly one prime factorisation. When the number is composite, that factorisation and the full list of divisors are shown.
Trial division up to the square root is used, which is exact and fast for numbers within JavaScript’s safe integer range.
Questions people ask
Is your figures uploaded to a server?
No. This tool runs entirely in your browser using standard web APIs, so your figures never leaves your device. You can confirm it by opening your browser's network panel while the tool runs, or by disconnecting from the internet after the page has loaded.
Is it free, and do I need an account?
Yes, it is free, and there is no account, no sign-up and no watermark on the result. The project is funded by optional donations.