Prime Number Checker
Check if a number is prime and see its factors if it is not.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Prime Number Checker
Enter any positive integer and the tool immediately tells you whether it is prime. For non-prime numbers, it lists all factors.
- Enter a number. Type any positive integer. The tool works with numbers up to the trillions.
- Read the result. "Prime" (in green) or "Not Prime" (in red) appears instantly. For composite numbers, the full list of factors is displayed below.
- Copy or share. Grab the result or send a link using the action buttons.
About Prime Numbers
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. The first few primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29. The number 2 is the only even prime, since every other even number is divisible by 2.
This tool uses trial division up to the square root of the input number. If no divisor is found up to that point, the number is prime. For composite numbers, all factor pairs are computed and displayed.
Frequently Asked Questions
Is 1 a prime number?
No. By definition, a prime number must be greater than 1. The number 1 has only one factor (itself), while primes must have exactly two distinct factors: 1 and the number itself.
Is 2 a prime number?
Yes. 2 is the smallest prime number and the only even prime. Every other even number is divisible by 2, so none of them can be prime.
How does the prime test work?
The calculator checks divisibility starting from 2 up to the square root of the number. If no divisor is found, the number is prime. This works because if a number has a factor larger than its square root, the corresponding pair factor must be smaller than the square root.