What is a prime number?

A prime number is a whole number greater than 1 that can only be divided evenly by 1 and itself. Numbers with more than two factors are called composite. 7 is prime because nothing except 1 and 7 divides it; 8 is composite because 2 and 4 also divide it.

Examples

• The primes under 20: 2, 3, 5, 7, 11, 13, 17, 19.
• 2 is the only even prime number.
• 9 is not prime because 3 × 3 = 9.

Quick facts

• 1 is not a prime number , it has only one factor.
• Every whole number can be written as a product of primes (prime factorisation).