Prime Number Checker

How it works

Enter a whole number and the tool checks divisibility by 2 and odd numbers up to its square root to determine primality, reporting the smallest factor when the number is composite. Enable the list option to generate every prime up to a capped limit.

Benefits

• Checks primality using efficient trial division
• Shows the smallest factor for composite numbers
• Can list every prime up to a chosen limit
• Runs instantly without uploading any numbers

Use cases

• Verify a number for a math assignment or puzzle
• Generate a list of primes for a cryptography exercise
• Explore factorization of composite numbers
• Check large candidate numbers quickly and privately

Best practices

• Use the list feature only when you need every prime, not just one check
• Keep list limits reasonable to avoid a very long output
• Remember 0, 1, and negative numbers are never prime
• Use single-number checks for very large candidates

Common mistakes

• Assuming 1 is a prime number
• Forgetting that even numbers other than 2 are never prime
• Requesting a prime list for a number far beyond the cap
• Expecting instant results for extremely large numbers

Privacy

All primality testing and sieve generation happen locally in your browser. Prime Number Checker does not transmit or store the numbers you enter.