- 81 is an odd composite number.
- 81 is the 9th squared number.
- 81 is the only number that is the square of the sum of its digits.
- In Greek-based number-naming systems, 81 is associated with prefix "henoctaconta-", and with prefix "unoctoginti-" in Latin systems.
- It can be named "garnine" with the gar- prefix.
- OEIS, Sequence A002808 (accessed 2021-02-07)