Prime Number Checker
Test for primes, find the next prime, or list them.
How to use
- 1 Choose whether to check a number, find the next prime, or list primes.
- 2 Enter your number or upper limit.
- 3 Read the result — including the smallest factor of a composite.
- 4 Copy the answer or the full list of primes.
About Prime Number Checker
The Prime Number Checker answers three related questions about prime numbers.
It tells you whether a given number is prime and, if it is not, the smallest factor that divides it; it finds the next prime strictly greater than any number you give; and it lists every prime up to a chosen limit.
A prime is a whole number greater than one with no divisors other than one and itself.
They are the building blocks of arithmetic and turn up constantly in cryptography, hashing, and number theory, but testing for them by hand gets tedious fast.
This tool uses trial division up to the square root for single checks, which is more than quick enough across the safe integer range, and a Sieve of Eratosthenes for listing, which is the efficient classical method for finding all primes below a bound.
When a number is composite, seeing its smallest factor is often more useful than a bare "not prime", so the checker reports it.
The listing mode is capped at one hundred thousand to keep memory bounded and the page responsive.
Zero and one are correctly treated as non-prime.
Everything runs locally in your browser, instantly and privately, and you can copy any result with a click.
FAQ
Are 0 and 1 prime?
No. A prime must be a whole number greater than one with exactly two divisors, so 0 and 1 are reported as not prime.
Why does it show a smallest factor?
When a number is composite, its smallest prime factor tells you why it is not prime and is often more useful than a plain "not prime".
Is there a limit on listing primes?
Yes, the list mode is capped at 100,000 to keep the calculation fast and memory use bounded.