81
Cardinal	eighty-one
Ordinal	eighty-first
Numeral system
Factorization
Roman numeral	LXXXI
Binary	01010001
Hexadecimal	51

Its factors are 1, 3, 9, 27, 81.

81 is:

• 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.