Prime Number Checker
Check whether a number is prime and find the next prime after it. Free and private, right in your browser.
Type in any whole number and instantly see whether it's prime, along with its factors if it isn't. Handy for checking homework, verifying a number for a cryptography or hashing exercise, or settling an argument about whether 91 is prime (it's not, 7 x 13).
How to use it
- Enter a whole number
- Hit check (or just watch it update as you type)
- See the prime/composite result and its factors
Frequently asked questions
- How large a number can I check?
- The tool handles arbitrarily large integers by trial division up to the square root, so numbers with a few dozen digits check instantly; truly huge numbers (100+ digits) will take longer since this isn't using a specialized primality test like Miller-Rabin.
- Is 1 a prime number?
- No. By definition a prime has exactly two distinct positive divisors, 1 and itself, so 1 is excluded and the tool will mark it as neither prime nor composite.
- What does it show for a composite number?
- It lists the full prime factorization (e.g. 84 = 2^2 x 3 x 7), not just a yes/no answer, so you can see why the number isn't prime.
- Does this send my number anywhere?
- No, the calculation runs entirely in your browser, so nothing you type is sent to a server.
- 1
Nothing is uploaded
The file never leaves your device. There is no server to send it to, so there is nothing to leak, log, or subpoena.
- 2
No signup, no limits
No account, no email, no daily cap, no watermark, and no paid tier waiting behind the third file.
- 3
Works offline
Load the page once and it keeps working with the network off, because all the work was always happening locally.