A prime number has exactly two divisors: 1 and itself. The checker tests divisibility up to the square root, shows the prime factorisation for composite numbers, and finds the previous and next primes.
How to use it
- Type a whole number.
- See whether it is prime and its factors.
Is 1 a prime number?
No. 1 has only one divisor. 2 is the only even prime.
Frequently asked questions
How big a number can it check?
Up to 9,007,199,254,740,991 (the largest exact integer in JavaScript).
Updated October 2026 · Built and tested by Sharjeel Tahir · How our tools are tested