🔢 Prime Number Checker

About this Prime Number Checker

This prime checker tests any whole number for primality by trial division and, when the number is not prime, lists every factor it has. Prime numbers — divisible only by 1 and themselves — are the atoms of arithmetic and the basis of modern encryption.

How to use it

1. Type the whole number you want to test.
2. Click Check number.
3. See whether it is prime, and if not, its complete list of factors.

Example

97 is prime, but 91 is not: 91 = 7 × 13, so its factors are 1, 7, 13 and 91.

Frequently asked questions

Is 1 a prime number?

No. Primes must have exactly two factors, and 1 has only one — so 1 is neither prime nor composite.

How large a number can it check?

It is instant for everyday numbers. Extremely large numbers (trillions and up) may take a moment because every possible divisor up to the square root is tested.

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