Factors of 81
Every whole number that divides 81 exactly, the factor pairs, and how to find them yourself.
The answer
The factors of 81 are 1, 3, 9, 27 and 81.
Factor pairs of 81
| Pair | Makes |
|---|---|
| 1 × 81 | 81 |
| 3 × 27 | 81 |
| 9 × 9 | 81 |
9 × 9 uses the same number twice, so 9 counts once. That is why 81 has an odd number of factors: 5.
Prime factors of 81
- Prime factorisation
- 81 = 3 × 3 × 3 × 3 (3⁴)
- Reading index form
- A small raised number tells you how many times to use that factor. For example, 2³ means 2 × 2 × 2.
- Is 81 prime?
- No. It has 5 factors, not just 1 and 81.
- Is 81 a square number?
- Yes. 9 × 9 = 81.
How to find the factors of 81
81 is odd, so every even number you try will fail. Its digits add up to 9, a multiple of 9, so 3 and 9 are factors. It is a square number, so one pair is 9 × 9, the same number twice. Work up from 1 to 9, and write down both numbers in every pair you find.
- 81 ÷ 1 = 81, so 1 × 81 is a pair.
- 2 does not go into 81 exactly.
- 81 ÷ 3 = 27, so 3 × 27 is a pair.
- 4 does not go into 81 exactly.
- 5 does not go into 81 exactly.
- 6 does not go into 81 exactly.
- 7 does not go into 81 exactly.
- 8 does not go into 81 exactly.
- 81 ÷ 9 = 9, so 9 × 9 is a pair.
Stop at 10: 10 × 10 = 100, which is more than 81. The next pair you would find is 27 × 3, which is 3 × 27 turned round, so you already have every pair.
81 in the times tables
9 × 9 = 81. Practise the 9 times table.
More numbers
Check whether 81 is prime or see the square root of 81.
Questions parents ask
What is the largest factor of 81, apart from 81 itself?
27. Divide 81 by its smallest factor above 1, which is 3: 81 ÷ 3 = 27.
What do the factors of 81 add up to?
121. Leaving out 81 itself, they add up to 40, which is less than 81, so 81 is called a deficient number.
Which number after 81 has the same number of factors?
625. Like 81, it has exactly 5 factors.
Continue your number practice
Explore the diagnostic for another set of practice questions.