🔢 Prime Number Checker
Is a number prime? Find all prime factors
About this calculator
Is that number prime, or does it have hidden factors? Enter any whole number and this checker tests divisibility up to the number's square root — the efficient way to test primality — then tells you whether it's prime and lists its factors if it's composite. It handles large numbers quickly and explains the reasoning behind the answer.
Tip: to test primes mentally, check divisibility by 2, 3, 5, then by primes up to the square root — you never need to test any further than that.