What is the Prime Number Checker?
A prime number is greater than 1 and divisible only by 1 and itself. This checker tests any whole number up to one trillion and shows a factor when it is composite.
How to use the Prime Number Checker tool
- Enter Number.
- The result updates instantly as you type, or press Calculate.
- Use Try an example to see how it works, and Reset to start again.
Formula
A number n is prime if no whole number from 2 up to √n divides it evenly.
Worked example
With number 91, the result comes to 91 is not prime. Smallest factor: 7; Cofactor: 13.
Frequently asked questions
Is 1 a prime number?
No. A prime must have exactly two distinct divisors, and 1 has only one.
Is 2 prime?
Yes. It is the only even prime number.
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Checked by Calcovate · Formula tested against worked examples · Updated 7 Nov 2026 · How we check our calculators.