91 is the natural number following 90 and preceding 92.

Cardinal	ninety-one
Ordinal	91st (ninety-first)
Factorization
Roman numeral	LXXVIII
Binary	1011011
Hexadecimal	4E

In mathematics

Ninety-one is a triangular number and a hexagonal number, one of the few such numbers to also be a centered hexagonal number, and it is also a centered nonagonal number and a centered cube number. It is the sum of the squares of the first six integers.

It is also the lowest natural number which looks prime but isn't, because neither of its factors 7 and 13 have simple divisibility tests.

It is the smallest positive integer expressible as a sum of two cubes in two different ways if negative roots are allowed: 91 = 6³+(-5)³ = 4³+3³. (See 1729 for more details).

It is also the smallest pseudoprime satisfying the congruence 3ⁿ = 3modn.

In science

• The atomic number of protactinium, an actinide.

In astronomy,

Messier object M91, a magnitude 11.5 spiral galaxy in the constellation Coma Berenices

The New General Catalogue object NGC 91 , a single star in the constellation Andromeda

The Saros number of the solar eclipse series which began on -159 August 6 and ended on 1175 October 16. The duration of Saros series 91 was 1334.2 years, and it contained 75 solar eclipses.

The Saros number of the lunar eclipse series which began on 161 May 26 and ended on 1459 July 15. The duration of Saros series 91 was 1298.1 years, and it contained 73 lunar eclipses.

In other fields

Ninety-one is also:

• The designation of American Interstate 91, a freeway that runs from Connecticut to Vermont.
• The code for international direct dial phone calls to India.
• In cents of a U.S. dollar, the amount of money one has if one has one each of the coins of denominations less than a dollar (penny, nickel, dime, quarter and half dollar).
• The ISBN Group Identifier for books published in Sweden.
• The year AD 91, 91 BC, or 1991.