Check whether a number is prime, and see its factors if not — free and instant.
A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. This checks a number by trial division — testing whether any number up to its square root divides it evenly, which is enough to conclusively prove primality without checking every number up to the input itself.
If the number isn't prime, its smallest prime factor is shown too, since that's usually the more useful piece of information when the answer is "no."
No — by definition, a prime number must have exactly two distinct divisors (1 and itself). Since 1’s only divisor is 1 itself, it’s excluded from the primes by definition, not by convention.
Yes — every other even number is divisible by 2, giving it at least three divisors (1, 2, and itself), so 2 is the sole exception.
Up to 1 quadrillion (1,000,000,000,000,000) — tested directly to stay fast at that size. Beyond that, finding the next/previous prime can take noticeably longer, so this tool asks for a smaller number instead of risking a slow, unresponsive check.
They’re shown regardless of whether the number you entered is itself prime — useful for finding the nearest prime above or below any number, not just checking one specific value.