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Prime Number Checker

Test whether a whole number is prime and see the nearest primes around it.

Test a number

Verdict: —
Smallest factor: —
Previous prime: — · Next prime: —
Numbers up to 1000000000000 are tested exactly by trial division.

How to use the Prime Number Checker

  1. Enter the whole number you want to test.
  2. Press Check to run the exact primality test.
  3. Read the verdict, the smallest factor and the nearest primes.

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Frequently Asked Questions

Common questions about the free online prime number checker.

Enter any whole number and press Check. The tool reports whether it is prime and names its smallest factor when it is not.
A prime has exactly two positive divisors: one and itself. Numbers with more divisors, such as 12, are composite.
One has only a single divisor, itself, so it does not meet the two divisor definition. It is neither prime nor composite.
Numbers up to one trillion are tested exactly by trial division, which finishes in well under a second in your browser.
They show the previous and next prime around your number, which is handy for quick checks and for choosing prime sizes.
Yes. Two is the only even prime, because every other even number is divisible by two as well as by one and itself.
No. The primality test runs entirely in your browser, so nothing you type is uploaded or stored anywhere.
Once the page has loaded it keeps working without a network connection, because all of the maths happens locally.

Last updated: October 10, 2026