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494 LearnersLast updated on August 5, 2025
Is 18 a Prime Number?
Why is 18 not a prime number?
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.
Using the Counting Divisors Method
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.
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Using the Divisibility Test Method
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.
Using Prime Number Chart
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
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
Common Mistakes to Avoid When Determining If 18 is a prime number
The chances of making a mistake while determining whether a number is prime or not are not too low. Here are some mistakes that can be made and the solutions to prevent them:
Mistake 1
Assuming 18 as a prime number instead of an even number
A kid might think that since 18 is divisible by 2, which is an even prime number, 18 is also a prime number. According to the definition that has been given, prime numbers are whole numbers with two factors. Here, 18 has 4 factors, so it won’t get identified as prime.
Mistake 2
Identifying numbers greater than 1 as prime numbers
Children can assume that numbers greater than 1 are prime. Always take a look at the definition of prime number when you are in doubt. The definition clearly says that any number with just two factors is called a prime number. Since numbers like 18 have more than two factors, they are non-prime.
Mistake 3
Always checking for small divisors
It is possible to stop finding the divisors after the smallest divisor has been found. Children often forget that a given number is also its divisor. For 18, the divisors are 1, 2, 9 and 18. Don’t stop checking for divisors at 2.
FAQs: Is18 a Prime Number?
1.Why is 2 the smallest even prime number?
2.Is 18 composite?
3.Can you list the prime numbers from 1 to 70?
4.What are the factors of 18?
Glossaries for “Is 18 a prime number?”
- 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.
Hiralee Lalitkumar Makwana
About the Author
Hiralee Lalitkumar Makwana has almost two years of teaching experience. She is a number ninja as she loves numbers. Her interest in numbers can be seen in the way she cracks math puzzles and hidden patterns.
Fun Fact
: She loves to read number jokes and games.
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