Is 1155 a Prime Number?
2026-02-28 11:04 Diff
https://brightchamps.com/en-us/math/numbers/is-1155-a-prime-number

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Last updated on August 5, 2025

The numbers that have only two factors, which are 1 and itself, are called prime numbers. Prime numbers are used in encryption, computer algorithms, and barcode generation. In this topic, we will be discussing whether 1155 is a prime number or not.

Is 1155 a Prime Number?

There are two types of numbers, mostly —

prime numbers and composite numbers, depending on the number of factors.

A prime number is a natural number that is divisible only by 1 and itself.

For example, 3 is a prime number because it is divisible by 1 and itself.

A composite number is a positive number that is divisible by more than two numbers.

For example, 6 is divisible by 1, 2, 3, and 6, making it a composite number.

Prime numbers follow a few properties like:

  • Prime numbers are positive numbers always greater than 1. 
     
  • 2 is the only even prime number. 
     
  • They have only two factors: 1 and the number itself. 
     
  • Any two distinct prime numbers are co-prime numbers because they have only one common factor, which is 1.

As 1155 has more than two factors, it is not a prime number.

Why is 1155 Not a Prime Number?

The characteristic of a prime number is that it has only two divisors: 1 and itself. Since 1155 has more than two factors, it is not a prime number. A few methods are used to distinguish between prime and composite numbers. These methods include: 

  • Counting Divisors Method 
     
  • Divisibility Test 
     
  • Prime Number Chart 
     
  • Prime Factorization

Using the Counting Divisors Method

The method in which we count the number of divisors to categorize the numbers as prime or composite is called the counting divisors method. Based on the count of the divisors, we categorize numbers as either prime or composite.

 If there is a total count of only 2 divisors, then the number is prime. 

If the count is more than 2, then the number is composite.

Let’s check whether 1155 is prime or composite.

Step 1: All numbers are divisible by 1 and themselves.

Step 2: Divide 1155 by 2. It is not divisible by 2, as it is an odd number.

Step 3: Divide 1155 by 3. The sum of the digits (1+1+5+5=12) is divisible by 3, so 1155 is divisible by 3.

Step 4: Continue this process up to the square root of 1155.

Step 5: When we divide 1155 by 3, 5, and 7, it is divisible by all.

Since 1155 has more than 2 divisors, it is a composite number.

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Using the Divisibility Test Method

We use a set of rules to check whether a number is divisible by another number completely or not. This is called the Divisibility Test Method. 

Divisibility by 2: 1155 is odd, so it is not divisible by 2.

Divisibility by 3: The sum of the digits in 1155 is 12, which is divisible by 3. 

Divisibility by 5: The unit’s place digit is 5, so 1155 is divisible by 5.

Divisibility by 7: Double the last digit (5 × 2 = 10), then subtract it from the rest of the number (115 - 10 = 105). Since 105 is divisible by 7, 1155 is also divisible by 7. 

Divisibility by 11: Alternating sum of digits (1 - 1 + 5 - 5 = 0), which is divisible by 11.

Since 1155 is divisible by 3, 5, 7, and 11, it has more than two factors. Therefore, it is a composite number.

Using Prime Number Chart

The prime number chart is a tool created by using a method called “The Sieve of Eratosthenes.” In this method, we follow these steps:

Step 1: Write numbers from 1 to 100 in 10 rows and 10 columns.

Step 2: Leave 1 without coloring or crossing, as it is neither prime nor composite.

Step 3: Mark 2 because it is a prime number and cross out all the multiples of 2.

Step 4: Mark 3 because it is a prime number and cross out all the multiples of 3.

Step 5: Repeat this process until you reach the table consisting of marked and crossed boxes, except 1. Through this process, we will have a list of prime numbers from 1 to 100.

1155 is not present in the list of numbers up to 100, so it is composite.

Using the Prime Factorization Method

Prime factorization is a process of breaking down a number into prime factors and then multiplying those factors to obtain the original number.

Step 1: We can write 1155 as 3 × 385.

Step 2: Break down 385 into 5 × 77.

Step 3: Further break 77 into 7 × 11.

Step 4: Now we have the product consisting of only prime numbers.

Hence, the prime factorization of 1155 is 3 × 5 × 7 × 11.

Common Mistakes to Avoid When Determining if 1155 is Not a Prime Number

Children might have some misconceptions about prime numbers when they are learning about them. Here are some mistakes that might be made by children.

FAQ on is 1155 a Prime Number?

1.Is 1155 a perfect square?

2.What is the sum of the divisors of 1155?

The sum of the divisors of 1155 is 2880.

3.What are the factors of 1155?

1155 is divisible by 1, 3, 5, 7, 11, 15, 21, 33, 35, 55, 77, 105, 165, 231, 385, and 1155, making these numbers the factors.

4.What are the closest prime numbers to 1155?

1151 and 1153 are the closest prime numbers to 1155.

5.What is the prime factorization of 1155?

The prime factorization of 1155 is 3 × 5 × 7 × 11.

Important Glossaries for "Is 1155 a Prime Number"

  • Composite numbers: Natural numbers greater than 1 that are divisible by more than 2 numbers are called composite numbers. For example, 1155 is a composite number because it is divisible by 1, 3, 5, 7, 11, 15, 21, 33, 35, 55, 77, 105, 165, 231, 385, and 1155. 
  • Prime numbers: Natural numbers greater than 1 that have no divisors other than 1 and themselves. For example, 7 is a prime number. 
  • Divisors: Numbers that divide another number completely without leaving a remainder. For example, the divisors of 12 are 1, 2, 3, 4, 6, and 12. 
  • Divisibility rules: Shortcuts used to determine if one number is divisible by another without performing division. For example, a number is divisible by 3 if the sum of its digits is divisible by 3.
  • Prime factorization: The process of expressing a number as the product of its prime factors. For example, the prime factorization of 1155 is 3 × 5 × 7 × 11.

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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.