Eighty-one (81, LXXXI) is the natural number following eighty and preceeding eighty-two.
- Hexadecimal: 51
- Binary: 01010001
- Factors: 1, 3, 9, 27, 81
- Factorization:
- Ordinal number: eighty-first
- the atomic number of thallium (Tl)
- the title of a short film by Stephen Burke: 81
- the model number of: Unterseeboot 81, Sinclair ZX81
- the Messier number of the M81 Group of galaxies
- the number of the French department Tarn
- the designation of Interstate 81, a freeway that runs from Tennessee to New York.
- the code for international direct dial phone calls to Japan
- the year AD 81 or AD 1981.