Prime Number Checker
Check primality and find neighbouring prime numbers.
Frequently Asked Questions
What is a prime number?
A whole number greater than 1 whose only divisors are 1 and itself. The first primes are 2, 3, 5, 7, 11 and 13. Note that 1 is neither prime nor composite by definition.
How does the checker test primality?
It uses trial division up to the square root of the number, skipping even candidates after testing 2. If no divisor is found, the number is prime, because any factor pair must include a factor at or below the square root.
Why do primes matter?
Primes are the building blocks of whole numbers and are essential to modern encryption: RSA security relies on the fact that multiplying two large primes is easy but factoring the result back is extremely hard.
Is the Prime Number Checker free, and is my data private?
Yes. It is completely free and every calculation runs locally in your browser, so the numbers and dates you enter are never uploaded, stored, or shared with anyone.
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