Prime Checker
Test primality and find a factor.
Enter the values and press Calculate.
How to use Prime Checker
A prime number has no divisors other than 1 and itself. This tool tests a number and, if composite, shows its smallest factor.
Open Prime Checker on toolpermarket. It runs entirely in your browser, so you can start without creating an account.
Enter the integer n to test in the fields above, then press Calculate.
Read the result below the form. Change any input and run it again as many times as you like — the math is instant and local.
Worked example. 17 has no divisor other than 1 and itself, so it is prime. 15 divides by 3, so it is not.
When to use Prime Checker. Use it in cryptography, factoring, or any check that needs to know if a number is prime.
Frequently asked questions
Is Prime Checker free to use?
Yes. Prime Checker runs entirely in your browser. There is no account, no upload, and no cost.
Does Prime Checker work on mobile?
Yes. The layout is responsive, so Prime Checker works on any phone, tablet, or desktop with a modern browser.
Is 1 a prime number?
No. By definition a prime has exactly two distinct positive divisors, 1 and itself. The number 1 has only one, so it is not prime.
How does the test work?
It tries dividing by every integer up to the square root of n. The first divisor found is reported as the smallest factor.