Prime Number Checker
Check whether any number is prime or composite
About this calculator
A prime number is divisible only by 1 and itself. To test if n is prime, check divisibility by all integers up to √n — if none divide evenly, n is prime. Examples: 2, 3, 5, 7, 11, 13... are prime. 4, 6, 9, 15 are composite (non-prime). Prime numbers are the building blocks of all integers.
How to use
Enter 97. Since no number from 2 to √97 ≈ 9.8 divides 97 evenly, it is prime. Enter 91: 91 = 7 × 13, so it is composite (a common trick question).
Frequently asked questions
Is 2 prime?
Yes. 2 is the only even prime number. Every other even number is divisible by 2, making it composite.
Are there infinitely many prime numbers?
Yes. Euclid proved this around 300 BC with an elegant proof by contradiction. Assume a finite list of primes, multiply them all and add 1 — the result is either prime or has a prime factor not in the list.