Prime Number Checker
Check whether a number is prime, with the factorisation and reasoning shown.
What is Prime Check?
A prime number has exactly two divisors, 1 and itself. Nothing else qualifies: 1 is not prime because it has only one divisor, and no composite number can be prime by definition. Primes are the building blocks of the integers, since every whole number above 1 factors into them uniquely.
How it works
Trial division runs up to the square root of the number and no further, because factors always pair up around it — if n = a × b with a ≤ b then a ≤ √n, so a factor must exist below the root if one exists at all. That bound is what makes the test practical, and the reason string says where it stopped. Composite answers name the factor that was found rather than only reporting "no".
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
Realistic sample
Type or paste any realistic input — the tool computes the result locally and instantly.
Common mistakes
Expecting a server round-trip
Nothing is uploaded — every tool in this category runs entirely in your browser.