Prime Number Checker

Check if a number is prime

The Prime Number Checker instantly tells you whether a whole number is prime. If it is not, it shows the prime factorization, and it always gives you the next prime that follows your number. Just type a number and the answer appears immediately — no formulas, no waiting.

What is a prime number?

A prime number is a whole number greater than 1 that has exactly two divisors: 1 and itself. The first primes are 2, 3, 5, 7, 11, and 13. Numbers with more than two divisors, like 12 (which divides by 1, 2, 3, 4, 6, and 12), are called composite. The number 2 is the only even prime, and 1 is neither prime nor composite by definition.

How the checker works

To test whether a number is prime, you only need to check for divisors up to its square root — if no divisor is found below that point, none exists above it either. This makes the check fast even for large numbers. When a number is composite, the tool breaks it down into its prime factors, the unique set of primes that multiply together to produce it.

How to use it

1. Type a whole number into the field.
2. See instantly whether it is prime or composite.
3. If composite, read its prime factorization; either way, see the next prime.

The first prime numbers

Range	Primes
1–10	2, 3, 5, 7
11–20	11, 13, 17, 19
21–30	23, 29

Why primes matter

• Cryptography — modern encryption like RSA relies on the difficulty of factoring large numbers into primes.
• Number theory — primes are the "building blocks" of all integers.
• Hashing — prime sizes help distribute values evenly.
• Education — a core concept in math classes worldwide.

Private and free

All checking happens in your browser, with nothing uploaded and no limits. Test as many numbers as you like, free.