Online Tool Store
Prime Number Checker
Check whether a number is prime, see its smallest factor if not, and list the five nearest primes below and above it, in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Instant for numbers up to about a trillion — much larger numbers may take a few seconds since this checks by trial division.
Nearby primes below
Nearby primes above
How the prime number checker works
- Enter a whole number.
- The tool checks divisibility by every number up to its square root to determine whether it's prime.
- It also lists the five nearest primes below and above your number, and the smallest factor if it isn't prime.
Why only check up to the square root?
If a number n has a factor larger than its square root, it must also have a matching factor smaller than the square root — so if no factor turns up by the time you reach √n, none exist at all. This cuts the amount of checking needed dramatically compared to testing every number up to n itself.
FAQ
Is 1 a prime number?
No — by definition, a prime number has exactly two distinct positive divisors (1 and itself). Since 1 only has one divisor, it's excluded from the primes by mathematical convention.
Why does it show the smallest factor for non-prime numbers?
Knowing the smallest factor is often more useful than just "not prime" — it shows exactly why the number isn't prime and gives you its simplest factorization pair.
How does the check work for very large numbers?
It tests for divisibility by every number up to the square root of your input (skipping even numbers after checking 2) — reliable and simple, though it can take a few seconds for extremely large numbers since there's more to check.
Is my number sent anywhere?
No. The primality check runs entirely in your browser — nothing is sent to Online Tool Store or any other server.
How we compare
| Feature | Online Tool Store | CalculatorSoup Prime Checker | WolframAlpha |
|---|---|---|---|
| Shows nearby primes too | ✓ | Prime check only | ✓ |
| Instant as you type | ✓ | Calculate button | Calculate button |
| No query limit | ✓ | ✓ | Limited free queries |
For an instant prime check with nearby primes shown alongside, the Online Tool Store Prime Number Checker gives you more context than a plain yes/no.