Calculators
Prime Number Checker
Check whether a whole number is prime and see its smallest factor locally in your browser.
Enter a whole number from 0 to 1000000000000 to check whether it is prime. Trial division runs locally in your browser; your value is not uploaded or stored.
Test a whole number for primality without uploading anything.
Enter a whole number from 0 to 1000000000000 to see whether it is prime. Composite numbers show their smallest factor pair. Everything runs locally in your browser, so your value is not uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What is a prime number?
A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. For example, 2, 3, 5, 7, 11, and 13 are prime. Numbers such as 0, 1, 4, 6, 8, and 9 are not prime.
How does this prime number checker work?
It checks divisibility with trial division: it first handles 2 and even numbers, then tests odd divisors up to the square root of the entered value. When a divisor is found, the tool reports the smallest factor pair; when none is found, the number is prime.
Which inputs are accepted?
Enter a whole number from 0 to 1000000000000 without decimals or extra characters. Values below 2 are reported as not prime, and values above the limit show a clear error so the browser check stays fast.
Do my numbers stay private?
Yes. The check and copy action run locally in your browser. CodeASystem does not upload or store the value you enter.