Category: Calculators
Prime Number Checker
Check whether a number is prime, instantly
Enter a whole number to check whether it is prime, and see its smallest factor if it isn't.
ResultPrime
A prime number has exactly two divisors: 1 and itself. 0, 1 and negative numbers are not prime.
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What this tool does
A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. This checker tests divisibility up to the square root of your number and reports the smallest factor it finds when the number is not prime.
How to use it
- Enter a whole number.
- Read whether it is prime.
- If it isn't, see its smallest factor other than 1.
Privacy
This tool runs entirely in your browser. Your input is never uploaded, stored or shared — closing the tab removes it.
Frequently asked questions
- Is 1 a prime number?
- No. By definition a prime number has exactly two distinct divisors, 1 and itself; 1 only has one divisor, so it is neither prime nor composite.
- Is 2 the only even prime number?
- Yes. Every other even number is divisible by 2 in addition to 1 and itself, so it has more than two divisors and cannot be prime.
- What is the largest number this can check?
- Up to 1,000,000,000,000 (one trillion). Beyond that, trial division would take too long to run smoothly in a browser tab.