Is 55 a prime number?
55 is not a prime number. Here is a way to check it.
The answer
No. 55 is not a prime number: 5 × 11 = 55.
How to see that 55 is not prime
55 ends in 5, and every number ending in 5 or 0 is in the 5 times table, so 5 is a factor.
55 has four factors: 1 and 55, plus one pair in between, 5 and 11.
Split 55 into prime factors
55 splits straight into two primes, 5 × 11, so its factor tree has only one branch.
55 in the times tables
55 turns up in 2 different times tables, the 5 and 11: 5 × 11 and 11 × 5.
Quick checks that catch 55
- It ends in 5, so 5 goes in.
- Its two digits match, and every two-digit number like that is in the 11 times table.
55 at a glance
Optional extra number facts:
- In words
- fifty-five
- In Roman numerals
- LV
- Odd or even
- Odd: it ends in 5
- Triangular number
- 1 + 2 + … + 10 = 55, a triangle of counters with 10 on the bottom row
- Its factors add up to
- 72, or 17 without 55 itself. 55 is deficient: Deficient means this sum is less than the number.
The numbers either side of 55
- 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
- 56 is not prime. 56 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 56. Check 56 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 55?
53 below it and 59 above it, a gap of 6.
What is 55 squared?
55 × 55 = 3025.
What is the smallest number with as many factors as 55?
6. It has 4 factors, the same as 55.
Continue your number practice
Explore the diagnostic for another set of practice questions.