Prime Number Checker | NumberUtils

How it works

A number greater than 1 is checked for any whole-number factor other than 1 and itself. If none exists, it is prime.

Formula: A prime number greater than 1 has no whole-number factors other than 1 and itself.

Examples

A prime number

17 is prime: no whole number between 2 and 16 divides evenly into it.

Not a prime number

18 is not prime: it divides evenly by 2, 3, 6 and 9, among others.

Things to keep in mind

0, 1 and negative numbers are not prime — this is a definition, not an error, so the calculator simply answers "no" for them.
2 is the only even prime number; every other even number has 2 as a factor.

Frequently asked questions

Is 1 a prime number?

No. By definition, a prime number must have exactly two whole-number factors: 1 and itself. 1 only has one factor (itself), so it is not prime.

What is the fastest way to check if a number is prime?

You only need to test whole-number divisors up to the square root of the number — if none of them divide evenly, none of the larger ones will either. This calculator uses exactly that method.