How to use the Prime Number Checker
- Enter a whole number.
- See whether it's prime.
- If not, read its prime factorization and smallest factor.
What does this tool do?
A prime number has exactly two factors: 1 and itself. 2, 3, 5, 7 and 11 are prime; 9 is not, because 9 = 3 × 3.
Every whole number above 1 can be written as a product of primes in exactly one way — its prime factorization. The checker finds it by trial division.
Why use it?
- Instant answers up to 10 trillion.
- Shows the full factorization, not just yes or no.
- Counts the divisors too.
Example: is 221 prime?
No. 221 = 13 × 17. It looks prime because it isn't divisible by 2, 3, 5, 7 or 11, which is why it's a classic trick question.
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Frequently asked questions
Is 1 a prime number?
No. A prime must have exactly two different factors, and 1 has only one.
Is 2 the only even prime?
Yes. Every other even number is divisible by 2, so it has at least three factors.
Last reviewed by the M2Toolkit team.