Prime Number Checker

Q: "What is a prime number?"

"A prime is a positive integer greater than 1 with no positive divisors other than 1 and itself (e.g. 2, 3, 5, 7)."

Q: "Is 1 prime?"

"No. By definition primes are greater than 1. 1 is neither prime nor composite."

Q: "What if the number is composite?"

"The calculator says \"Composite\" and may show a small factor (e.g. 2, 3, 5) if found quickly."

Q: "How large can the number be?"

"Very large numbers may take longer. The tool uses trial division so very large primes may not be certified quickly."

Q: "What about negative numbers?"

"Primality is usually defined for positive integers. The tool typically uses the absolute value or asks for a positive integer."

Check if a number is prime. Example: 17 → Yes, prime. Shows small factors if composite. Copy result.

Prime numbers are positive integers greater than 1 with no divisors other than 1 and themselves. Enter an integer. The calculator says whether it is prime or composite and may show a small factor. Copy the result or Copy with labels.

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Examples

17 → Prime
15 → Composite (e.g. 3 × 5)
2 → Prime

FAQ

What is a prime number?

A prime is a positive integer greater than 1 with no positive divisors other than 1 and itself (e.g. 2, 3, 5, 7).

Is 1 prime?

No. By definition primes are greater than 1. 1 is neither prime nor composite.

What if the number is composite?

The calculator says "Composite" and may show a small factor (e.g. 2, 3, 5) if found quickly.

How large can the number be?

Very large numbers may take longer. The tool uses trial division so very large primes may not be certified quickly.

What about negative numbers?

Primality is usually defined for positive integers. The tool typically uses the absolute value or asks for a positive integer.