Prime Number Calculator

Use this prime number calculator to verify if a number is prime or not. If the number that you entered is composite, the calculator will tell you by what factor your number is divisible.

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Facts about Prime and Composite Numbers

By definition a number is prime if it is divisible only by one and itself. The only two factors of a prime number are one and the number itself.

Except for 0 and 1 (who are neither prime, nor composite), a whole number that is not prime is composite. A composite number has other factors besides one and itself. Two is the only even prime number. Any other even number is composite because it has two as a factor.


The following numbers are prime: 2, 3, 5, 7, 11, 13, 17, 19,...

The following numbers are composite: 4, 6, 8,9, 10, 12, 14, 15, 16, 18, 20

Use the above calculator to establish whether a large number is prime or composite.

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