A prime number has exactly two divisors: 1 and itself. This tool tells you instantly whether a number is prime, and if it is not, it shows the full prime factorization — the prime numbers that multiply together to form it. It also lists every prime in a range you choose, which is handy for studying number theory, homework or quick math checks.
All calculations run locally in your browser, so large numbers are processed instantly and nothing is sent to a server. It is free, needs no account and works offline once the page is loaded.
No. By definition a prime has exactly two distinct positive divisors, so 1 (with a single divisor) is neither prime nor composite.
For a single check it handles numbers up to about 10 trillion accurately using trial division.
If a number has a divisor greater than its square root, it must also have a smaller one, so testing up to the square root is enough.
Disclaimer: Results are mathematically standard but verify critical calculations independently.