Prime Number Checker
Check whether a number is prime, with its smallest factor if it is not.
Quick answer: enter your details below and click Check Primality — your result appears instantly, right here in your browser. Nothing you enter is uploaded or stored.
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How to use
- Enter a whole number.
- Click Check Primality.
- Read whether it is prime, or its smallest factor if it is not.
About this tool
Enter a whole number and the tool tests it by trial division up to its square root. If the number is not prime, it reports the smallest factor it found, so you can see why. Numbers below 2 are not prime by definition.
Frequently asked questions
Is 1 prime?
No. Primes must have exactly two factors, and 1 has only itself.
Is 2 prime?
Yes. It is the only even prime number.