Prime Number Calculator
Check whether a number is prime — and if not, get its prime factorisation.
How to use the Prime Number Calculator
- Enter any whole number.
- See instantly whether it is prime or composite.
- For composite numbers, view the smallest factor and full prime factorisation.
A prime number greater than 1 is divisible only by 1 and itself.
About the Prime Number Calculator
A prime number is a whole number greater than 1 that has exactly two divisors: 1 and itself. 2, 3, 5, 7, 11 and 13 are the first few. Every other whole number is composite — it can be broken into a product of primes.
This tool tells you at a glance whether a number is prime. If it isn’t, it shows the smallest factor and the complete prime factorisation — the unique set of primes that multiply to give your number (for example, 60 = 2 × 2 × 3 × 5).
Primes are the building blocks of arithmetic and the backbone of modern encryption. Use this for homework, curiosity, or for simplifying fractions and finding lowest common denominators.
Frequently asked questions
Is 1 a prime number?
No. A prime has exactly two distinct divisors, but 1 has only one. 2 is the smallest prime — and the only even one.
What is prime factorisation?
It’s writing a number as a product of primes, such as 84 = 2 × 2 × 3 × 7. Every whole number above 1 has one unique prime factorisation.
How large a number can I check?
The tool checks numbers into the billions instantly. Extremely large numbers may take a moment but still work.