Home › Calculators › Prime Number Calculator
Enter any whole number to test primality. Composite numbers get their prime factorisation; enter 1,000,000 or less and you also get the count of primes below it and the largest one — all computed with an exact sieve, never guessed.
A prime has exactly two divisors: 1 and itself. 2 is the only even prime; 1 is not prime at all.
Trial division up to the square root for the primality test, and the Sieve of Eratosthenes for listing — the same algorithms textbooks teach, run exactly.
Modern encryption (RSA) multiplies huge primes together — easy to multiply, practically impossible to factor back. Your bank login leans on primes.
A whole number greater than 1 divisible only by 1 and itself: 2, 3, 5, 7, 11…
No — primes need exactly two divisors, and 1 has only one.
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