Is 90 a prime number?
90 is not a prime number. Here is a way to check it.
The answer
No. 90 is not a prime number: 2 × 45 = 90.
How to see that 90 is not prime
90 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 90.
90 has 12 factors (1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45 and 90), which make 6 factor pairs.
Split 90 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 90 = 2 × 45
- 90 = 2 × 3 × 15
- 90 = 2 × 3 × 3 × 5
With repeated primes written as powers, that is 2 × 3² × 5. A small raised number tells you how many times to use that factor: for example, 2³ means 2 × 2 × 2.
90 in the times tables
90 turns up in 2 different times tables, the 9 and 10: 9 × 10 and 10 × 9.
Quick checks that catch 90
- The last digit, 0, is even, so 2 goes in.
- The digits add up to 9, which is in the 3 times table, so 3 goes in.
- It ends in 0, so 5 goes in.
- It is even and passes the check for 3 as well, so 6 goes in.
- The digit total, 9, is a multiple of 9, so 9 goes in.
- It ends in 0, so 10 goes in.
90 at a glance
Optional extra number facts:
- In words
- ninety
- In Roman numerals
- XC
- Odd or even
- Even: it ends in 0
- Multiple of 10
- 9 tens
- Its factors add up to
- 234, or 144 without 90 itself. 90 is abundant: Abundant means this sum is greater than the number.
The numbers either side of 90
- 89 is prime: 2, 3, 5 and 7 do not go into it, and 11 × 11 is already past 89. Check 89 in full
- 91 is not prime. 91 is odd, its digits do not add up to a multiple of 3, and it does not end in 5, so the quick checks for 2, 3 and 5 all fail. Try 7 next: 7 × 13 = 91. Check 91 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 90?
89 below it and 97 above it, a gap of 8.
What is 90 squared?
90 × 90 = 8100.
What is the smallest number with as many factors as 90?
60. It has 12 factors, the same as 90.
Continue your number practice
Explore the diagnostic for another set of practice questions.