81
81 is both a perfect square (9²) and a fourth power (3⁴), and the atomic number of thallium.
Where 81 fits
Why 81 is special
81's dual identity — square and fourth power at once — is a genuine mathematical curiosity most two-digit numbers don't share.
81 is part of
Factors & digit properties
- Divisors (5)
- 1, 3, 9, 27, 81
- Prime factorisation
- 3^4
- Digit sum
- 9
- Digital root
- 9
Representations
- Decimal
- 81
- Binary
- 1010001
- Octal
- 121
- Hexadecimal
- 51
- Roman numeral
- LXXXI
- Scientific notation
- 8.100 × 10^1
Other mathematical patterns
- Deficient number
- Powerful number
- Harshad (Niven) number
Related numbers & connections
Mathematical relationship81 is the square of 9 (9² = 81).
81 is the square of 981 is the square of 9 (9² = 81).
81 is the square of 9 (9² = 81).
View 9 →Mathematically related
- is the square of9
why?
Strength 0.50, confidence "high".
Science
81 is the atomic number of thallium.
DocumentedScience
Mathematics
81 = 9² = 3⁴ — both a perfect square and a fourth power.
DocumentedMathematics
Common questions
Is 81 a prime number?
No, 81 is not prime.
What are the factors of 81?
The factors of 81 are 1, 3, 9, 27, 81.
What is 81 in Roman numerals?
81 in Roman numerals is LXXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026