Check whether a number is prime instantly, using an efficient square-root-based check, right in your browser.
Enter a number to check whether it's prime.
A prime number is a whole number greater than 1 that has exactly two positive divisors: 1 and itself. Every other whole number greater than 1 is called composite, meaning it can be evenly divided by at least one number other than 1 and itself.
| Number | Result | Why |
|---|---|---|
| 2 | Prime | Only divisible by 1 and 2 |
| 3 | Prime | Only divisible by 1 and 3 |
| 4 | Not Prime | Also divisible by 2 (2 × 2) |
| 17 | Prime | Only divisible by 1 and 17 |
| 1 | Not Prime | Only one divisor (itself), not two |
Rather than testing every number up to the number itself, this tool only tests potential divisors up to its square root. If a number has a factor larger than its square root, that factor must be paired with a smaller one — one that would already have been found during the check. This cuts the work from checking millions of possible divisors down to checking only a few thousand, even for a fairly large number, which is why the result appears instantly rather than making your browser freeze.
A prime number is a whole number greater than 1 that has exactly two positive divisors: 1 and itself. 7 is prime because only 1 and 7 divide it evenly; 8 is not, because 1, 2, 4, and 8 all divide it evenly.
A prime number needs exactly two distinct positive divisors. 1 only has one divisor — itself — so it doesn't qualify, by definition.
Primality is only defined for whole numbers greater than 1. 0 and negative numbers fall outside that range entirely.
No — primality only applies to whole numbers (integers). A decimal input is rejected with a clear message rather than checked incorrectly.
It only tests potential divisors up to the square root of the number, since any factor larger than the square root would have to be paired with one smaller than it (which would already have been found). This makes checking a number with 15 digits take a fraction of a second instead of an impractically long time.
Yes — up to 9,007,199,254,740,991 (2^53 − 1), the largest integer JavaScript can represent with full precision. Beyond that, floating-point numbers start skipping some integers entirely, so the result could be checking the wrong number without any way to tell. This tool clearly rejects anything larger rather than risk a silently wrong answer.
No. Every check happens locally in your browser using JavaScript. Nothing you enter here is sent to Brekzy or stored anywhere.
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