Is 81 a prime number?
81 is not a prime number. Here is a way to check it.
The answer
No. 81 is not a prime number: 3 × 27 = 81.
How to see that 81 is not prime
Its digits add up to 8 + 1 = 9, which is a multiple of 3. That means 81 is a multiple of 3 too, so 3 is a factor.
81 has 5 factors (1, 3, 9, 27 and 81), which make 3 factor pairs.
Split 81 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 81 = 3 × 27
- 81 = 3 × 3 × 9
- 81 = 3 × 3 × 3 × 3
With repeated primes written as powers, that is 3⁴. A small raised number tells you how many times to use that factor: for example, 2³ means 2 × 2 × 2.
81 in the times tables
Just one of the 2 to 12 times tables has 81 in it: 9 × 9 = 81.
Quick checks that catch 81
- The digits add up to 9, which is in the 3 times table, so 3 goes in.
- The digit total, 9, is a multiple of 9, so 9 goes in.
81 at a glance
Optional extra number facts:
- In words
- eighty-one
- In Roman numerals
- LXXXI
- Odd or even
- Odd: it ends in 1
- Square number
- 9 × 9 = 81
- Its factors add up to
- 121, or 40 without 81 itself. 81 is deficient: Deficient means this sum is less than the number.
The numbers either side of 81
- 80 is not prime. 80 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 80. Check 80 in full
- 82 is not prime. 82 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 82. Check 82 in full
Turn the digits round
81 backwards is 18. 18 is not prime. 18 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 18. Check 18 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 81?
79 below it and 83 above it, a gap of 4.
What is 81 squared?
81 × 81 = 6561.
What is the smallest number with as many factors as 81?
16. It has 5 factors, the same as 81.
Continue your number practice
Explore the diagnostic for another set of practice questions.