What does this tool do?
This prime checker instantly tells you whether any whole number is prime, and if it isn't, breaks it down into its prime factors. It also lists every prime up to a limit you choose, making it handy for students, teachers and anyone exploring number theory.
How to use it
- Enter your values into the fields.
- The result is calculated instantly.
- Read or copy the result.
What makes a number prime?
A prime is a whole number greater than 1 that is divisible only by 1 and itself. The tool tests divisors up to the square root of your number, which is why it stays fast even for large inputs: no factor can hide beyond that point. Two is the only even prime; every prime after it is odd. Negative values and decimals are reduced to the nearest whole number so you always get a meaningful answer.
Factorization and list mode
When a number is not prime, the tool shows it as a product of its prime factors, for example 84 = 2 × 2 × 3 × 7. That is useful for simplifying fractions, finding GCD/LCM and working through cryptography exercises. The list tab uses the Sieve of Eratosthenes to generate every prime up to your chosen limit (capped at 10,000), which you can copy with one click for homework or code.
Frequently asked questions
Is 1 a prime number?
No. A prime must have exactly two distinct divisors; 1 has only one, so it is neither prime nor composite.
How high can the prime list go?
List mode generates primes up to 10,000 to keep it instant. The single-number check can test far larger values quickly.
Can the factor result repeat the same number?
Yes. The factorization lists prime factors with their multiplicity, e.g. 12 = 2 × 2 × 3. The badges above show each distinct prime factor only once.
Is Prime Number Checker free?
Yes. Prime Number Checker and all 300+ tools on MagmaNex are completely free with no signup or account required.
Is my data safe?
Yes. Everything runs locally in your browser. The data you enter is never sent to a server, stored, or shared.
Does it work on mobile?
Yes. Prime Number Checker works on phones, tablets and desktops — no app download needed.
🔒 Privacy: everything runs in your browser, your data is never sent to a server.