Is 18 a Prime Number?

It is not possible to say that the number 18 is prime. When we find the factors of 18, we get 1, 2, 9 and 18 as its factors, which makes it composite. 

The whole number having just two factors falls into the prime number category. Here, number 18 is a composite number having 4 factors in total. Hence, 18 is a non-prime number.

In order to see whether a number is prime or not, follow these methods:

• Counting Divisors Method

• Divisibility Test

• Prime Number Chart

• Prime Factorization

Now let’s find out why 18 is not a prime number.
 

The counting divisors method is used to find the total number of divisors a number has and determine whether a number is prime or not based on the number of divisors.

For number 18,

Divisors = 1, 2, 9, 18

Total number of divisors = 4

Therefore, 18 is a non-prime number.

The divisibility test is for prime numbers. Since 18 is a composite number, there’s no divisibility test. All we need to do is check for numbers other than 1 and 18 that divide 18 completely.

 Number 18 is completely divisible by numbers 2 and 9

18 ÷ 2 = 9

18 ÷  9 = 2

Hence, we can say that 18 is divisible by 2 and 9, not just 1 and 18 itself.
 

With the help of a prime number chart, we can verify whether a given number is prime or not.
Check the chart given below and see if 18 is present:

Since we cannot spot 18 in the prime number chart given above, we can say that 18 is not a prime number.
 

Prime factorization is applicable for numbers that are categorized as non-prime. Since 18 is a composite number, it can be prime factorized. This method prime factorizes 18 into its prime numbers. 

The prime factorization of 18 is 2 × 3 × 3 → 2¹ × 3²
 

• Prime Number: A natural number having 1 and the number itself as factors.

• Composite number: Non-prime numbers that can be divided into more than two factors.

• Prime Factorization: The process of dividing the given number into its prime factors.