Prime Number Checker Calculators
Check whether a number is prime and see its factorisation.
Prime Number Checker is a free online tool that check whether a number is prime and see its factorisation. It runs entirely in your web browser using plain JavaScript, so files are processed on your own device and never uploaded to a server. There is no sign-up, no file size limit imposed by the site, no watermark and no paid tier.
- Price
- Free — no account, no quota, no watermark
- Category
- Calculators
- Where it runs
- In your browser, on your device
- Files uploaded
- None
- Technology
- plain JavaScript
- Settings
- 1
- Works offline
- Yes, after the first visit
About Prime Number Checker
Primality is tested by trial division up to the square root of the number, which is enough because any factor above the square root must pair with one below it. Factorisation uses the same bound. The tool caps input at a trillion so the check stays instantaneous — beyond that you want a probabilistic test like Miller–Rabin.
How to use it
- Enter number.
- Results update the moment anything changes.
The one setting
| Setting | What it does | Default |
|---|---|---|
| Number | A numeric value. | 9973 |
Under the hood
| Runs on | plain JavaScript — runs the whole thing |
| Controls | Number |
Questions
Is 1 a prime number?
No. A prime has exactly two distinct divisors, and 1 has only one. Excluding it is what makes prime factorisation unique — otherwise you could pad any factorisation with as many 1s as you liked.
Why does the tool slow down on very large numbers?
Trial division does about √n operations. At a trillion that is a million checks, which is still fast; at 10^18 it would be a billion, which is not. Cryptographic primality testing uses a different approach entirely.
Is the arithmetic exact?
It uses double-precision floating point — about 15 significant figures, far more than the inputs justify. Displayed rounding is cosmetic; the underlying numbers are not rounded between steps, so chained results do not drift.