Prime Number Checker
Check if a number is prime and find all primes up to any limit.
How to Use the Prime Number Checker
This tool lets you check whether a number is prime and find all primes up to any limit:
- Choose a mode — use "Check a Number" to test a single number for primality, or "Find Primes Up To N" to list all primes up to a given limit.
- Enter a number — type any positive integer into the input field.
- Click the button — in check mode the tool tells you whether the number is prime and, if not, lists its factors. In find mode it lists every prime up to your number.
- Copy the results — in find mode, click "Copy All" to copy the full list of primes to your clipboard.
Everything runs in your browser — no data is sent to any server.
Frequently Asked Questions
What is a prime number?
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 2, 3, 5, 7 and 11 are the first five primes.
Is 1 a prime number?
No. By definition, a prime number must be greater than 1. The number 1 has only one positive divisor (itself), so it does not meet the requirement of having exactly two distinct positive divisors.
How large a number can I check?
You can check any integer your browser can handle accurately (up to about 9 quadrillion). In find mode the limit is 10,000,000 to keep the browser responsive.
What algorithm does this tool use?
For checking a single number the tool uses trial division up to the square root. For finding all primes up to N it uses the Sieve of Eratosthenes, one of the most efficient classical algorithms for this purpose.
Are the results accurate?
Yes. Both algorithms are mathematically exact and produce correct results for all valid inputs within the supported range.
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