Is 45 a prime number?
45 is not a prime number. Here is a way to check it.
The answer
No. 45 is not a prime number: 3 × 15 = 45.
How to see that 45 is not prime
Its digits add up to 4 + 5 = 9, which is a multiple of 3. That means 45 is a multiple of 3 too, so 3 is a factor.
45 has 6 factors (1, 3, 5, 9, 15 and 45), which make 3 factor pairs.
Split 45 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 45 = 3 × 15
- 45 = 3 × 3 × 5
With repeated primes written as powers, that is 3² × 5. A small raised number tells you how many times to use that factor: for example, 2³ means 2 × 2 × 2.
45 in the times tables
45 turns up in 2 different times tables, the 5 and 9: 5 × 9 and 9 × 5.
Quick checks that catch 45
- The digits add up to 9, which is in the 3 times table, so 3 goes in.
- It ends in 5, so 5 goes in.
- The digit total, 9, is a multiple of 9, so 9 goes in.
45 at a glance
Optional extra number facts:
- In words
- forty-five
- In Roman numerals
- XLV
- Odd or even
- Odd: it ends in 5
- Triangular number
- 1 + 2 + … + 9 = 45, a triangle of counters with 9 on the bottom row
- Its factors add up to
- 78, or 33 without 45 itself. 45 is deficient: Deficient means this sum is less than the number.
The numbers either side of 45
- 44 is not prime. 44 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 44. Check 44 in full
- 46 is not prime. 46 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 46. Check 46 in full
Turn the digits round
45 backwards is 54. 54 is not prime. 54 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 54. Check 54 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 45?
43 below it and 47 above it, a gap of 4.
What is 45 squared?
45 × 45 = 2025.
What is the smallest number with as many factors as 45?
12. It has 6 factors, the same as 45.
Continue your number practice
Explore the diagnostic for another set of practice questions.