No, 81 is not a prime number. It is a composite number: its digits add up to 9, which is divisible by 3, so 81 is divisible by 3.
81 has divisors other than 1 and itself. The smallest is 3: 81 ÷ 3 = 27.
Prime factorization: 81 = 3 × 3 × 3 × 3 = 3⁴.
| Number | Prime? |
|---|---|
| 77 | No, 7 × 11 |
| 78 | No, 2 × 3 × 13 |
| 79 | Yes |
| 80 | No, 2 × 2 × 2 × 2 × 5 |
| 82 | No, 2 × 41 |
| 83 | Yes |
| 84 | No, 2 × 2 × 3 × 7 |
| 85 | No, 5 × 17 |
More about 81: factors of 81 · 81 squared · square root of 81 · 81 in binary · 81 in Roman numerals.
Another number? Use the prime number checker for any value, or see the list of prime numbers.