Names of 81
| Nominal | 81 |
| Cardinal | eighty-one |
| Ordinal | 81st |
| This number as US currency | eighty-one dollars |
| Is 81 an odd number? | 81 is an odd number |
| Is 81 an even number? | 81 is NOT an even number |
| Is 81 a palindrome? | 81 is NOT a palindrome number |
| Is 81 a triangle number? | 81 is NOT a triangle number |
81 in other Bases
| Base 2 (Binary) | 10100012 |
| Base 3 (Ternary) | 100003 |
| Base 4 (Quaternary) | 11014 |
| Base 5 (Quinary) | 3115 |
| Base 6 (Senary) | 2136 |
| Base 8 (Octal) | 1218 |
| Base 12 (Duodecimal) | 6912 |
| Base 16 (Hexadecimal) | 5116 |
| Base 20 (Vigesimal) | 4120 |
Primes and Factors tests for 81
| Is 81 a Prime Number? | 81 is NOT a prime |
| Is 81 a perfect number? | 81 is NOT a perfect number |
| Is 81 a Mersenne Prime? | 81 is NOT a Mersenne prime |
| Is 81 a Fermat Prime? | 81 is NOT a Fermat prime |
| Is 81 a Fibonacci Prime? | 81 is NOT a Fibonacci prime |
| Is 81 a Partition Prime? | 81 is NOT a Partition prime |
| Is 81 a Pell Prime? | 81 is NOT a Pell prime |
| Prime Factors | Prime factors of 81 |
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81 in other Numerals
| 81 in Roman Numeral | LXXXI |
| 81 in Chinese Numeral | 捌拾壹 |
| 81 in Egyptian Numeral | 𓎍𓏺 |
| 81 in Babylonian Numeral |   |
Roots and stuff for 81
| Is 81 a square number? | 81 is a square number |
| Is 81 a cube number? | 81 is NOT a cube number |
| Square root of the number 81 | 9 |
| Cube root of the number 81 | 4.3267487109222 |
Recreational math for 81
| Number 81 reversed | 18 |
| Sum of the digits | 9 |
| No of digits | 2 |