Prime Number Checker
Check whether a number is prime, and see why.
Is it prime?
A prime number is a whole number greater than one whose only divisors are one and itself. This checks whether a given number is prime, and if not, shows a factor that divides it, so you can see not just the answer but the reason.
Enter a number to check whether it is prime.
Why primes are special
Primes are the building blocks of the whole numbers — every integer greater than one is either prime or a unique product of primes, which is why they are called the atoms of arithmetic. Checking primality gets computationally harder as numbers grow very large, and that difficulty is not just academic: much of modern encryption relies on the fact that multiplying two large primes is easy but factoring the result back into them is extremely hard. So the humble question of whether a number is prime sits at the foundation of both pure number theory and the security of everyday digital communication, which is a lot of weight for such a simple definition to carry.
Prime Number Checker FAQ
What makes a number prime?
It is a whole number greater than one whose only divisors are one and itself — so it cannot be evenly divided by any other number.
Why do primes matter beyond maths class?
They underpin modern encryption, which relies on multiplying large primes being easy while factoring the result back is extremely hard.
Is anything uploaded?
No. The check happens in your browser.