Free Prime Number Checker
Type any number to find out instantly whether it is prime — plus its prime factorization and the closest primes above and below it.
Prime Number Checker
Is it prime? Plus factors and the nearest primes.
Add this free tool to your website
Embed the prime checker on your classroom site, blog, or math resource. It is free and works anywhere. Just copy this snippet and paste it into your page:
<iframe src="https://www.prime59.fun/widget/prime-number-checker" width="100%" height="520" style="border:none;max-width:600px" loading="lazy" title="Free Prime Number Checker"></iframe>
<p style="font:13px sans-serif">Free <a href="https://www.prime59.fun">prime number checker</a> by Prime 59</p>Tip: it drops straight into WordPress, Webflow, Squarespace, or any HTML page.
What is a prime number?
A prime number is a whole number greater than 1 that has exactly two factors: 1 and itself. 2, 3, 5, 7, and 11 are prime; 4, 6, 8, and 9 are composite because they can be divided evenly by smaller numbers. The number 1 is neither prime nor composite. Primes are the building blocks of every other whole number.
How do you check if a number is prime?
The basic method is trial division: test whether the number divides evenly by any integer from 2 up to its square root. If none do, it is prime. This checker uses a fast, exact primality test so even large numbers return instantly, and it shows the prime factorization for composite numbers.
Why prime numbers matter
Primes power modern encryption, hashing, and random-number generation, and they are a favorite of puzzle designers because their patterns are simple to state but hard to predict. If you enjoy spotting primes, Prime 59 turns that instinct into a game.