Is 15 a prime number?
15 is not a prime number. Here is a way to check it.
The answer
No. 15 is not a prime number: 3 × 5 = 15.
How to see that 15 is not prime
Its digits add up to 1 + 5 = 6, which is a multiple of 3. That means 15 is a multiple of 3 too, so 3 is a factor.
15 has four factors: 1 and 15, plus one pair in between, 3 and 5.
Split 15 into prime factors
15 splits straight into two primes, 3 × 5, so its factor tree has only one branch.
15 in the times tables
15 turns up in 2 different times tables, the 3 and 5: 3 × 5 and 5 × 3.
Quick checks that catch 15
- The digits add up to 6, which is in the 3 times table, so 3 goes in.
- It ends in 5, so 5 goes in.
15 at a glance
Optional extra number facts:
- In words
- fifteen
- In Roman numerals
- XV
- Odd or even
- Odd: it ends in 5
- Triangular number
- 1 + 2 + … + 5 = 15, a triangle of counters with 5 on the bottom row
- Its factors add up to
- 24, or 9 without 15 itself. 15 is deficient: Deficient means this sum is less than the number.
The numbers either side of 15
- 14 is not prime. 14 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 14. Check 14 in full
- 16 is not prime. 16 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 16. Check 16 in full
Turn the digits round
15 backwards is 51. 51 is not prime. Its digits add up to 5 + 1 = 6, which is a multiple of 3. That means 51 is a multiple of 3 too, so 3 is a factor. Check 51 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 15?
13 below it and 17 above it, a gap of 4.
What is 15 squared?
15 × 15 = 225.
What is the smallest number with as many factors as 15?
6. It has 4 factors, the same as 15.
Continue your number practice
Explore the diagnostic for another set of practice questions.