Check if an integer is prime and inspect a factor for composite numbers.
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Prime
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In-depth guide
A prime number is an integer greater than 1 whose only divisors are 1 and itself — 2, 3, 5, 7, 11, 13… Every other integer above 1 is composite. This tool tests an integer for primality and, when it is composite, shows a divisor, all in your browser.
To check whether n is prime you only need to test divisors up to √n. If no integer from 2 up to the square root of n divides it evenly, n is prime. The square-root shortcut works because any factor larger than √n must pair with one smaller than √n, so a smaller divisor would already have been found.
Primes are the atoms of number theory and the foundation of modern cryptography — RSA security rests on the difficulty of factorising the product of two large primes. This checker is sized for learning and exercises rather than the astronomically large numbers real cryptography uses, where probabilistic tests like Miller–Rabin take over.
The test runs locally in your browser — nothing is uploaded. By definition 0 and 1 are not prime, and 2 is the only even prime. Decimals are not supported, since primality is defined for whole numbers.
Use this calculator for quick everyday estimates, date math, percentages, bill splitting, or simple planning. Use an official source or domain-specific calculator when legal, billing, payroll, or compliance accuracy is required.
Integers greater than 1 whose only divisors are 1 and themselves — 2, 3, 5, 7, 11, and so on.
No. Primality is defined only for whole numbers, so this checker takes integers.
Yes. For composite numbers the smallest factor is shown so you can start factorising.
Any factor larger than the square root pairs with a smaller one, so if no divisor exists below the square root, the number is prime.
No. By definition primes are greater than 1, so 0 and 1 are neither prime nor (for 1) composite. 2 is the only even prime.
Real cryptography uses astronomically large numbers and probabilistic tests like Miller-Rabin. This tool uses straightforward trial division for learning-scale numbers.
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