Prime Number Checker
Check if a number is prime and see all its factors.
What is a prime number checker?
A prime number checker tells you whether a number is prime. A prime number has only two factors: 1 and itself. Example: 17 is a prime number.
How to use it
Enter a number, then tap Check. The factors and working steps are shown below the answer.
Why test divisors up to the square root?
To test 17, only check divisors up to √17 ≈ 4.12, that is 2 and 3. Neither divides 17, so 17 is prime. For 18, 18 ÷ 2 = 9, so 18 is not prime.
Example
17 has factors 1 and 17 only. Testing divisors 2 and 3 (up to √17 ≈ 4.12) shows neither divides 17, so 17 is prime. By contrast, 18 has factors 1, 2, 3, 6, 9 and 18, so it is not prime.
Frequently Asked Questions
Is 17 a prime number?
Yes, 17 is prime.
Is 1 a prime number?
No, prime numbers are 2 or greater.
Is 18 a prime number?
No, 18 ÷ 2 = 9, so 2 is a divisor.
More calculators: Math Tools and All Tools.