66 is the natural number following 65 and preceding 67.

Cardinal	sixty-six
Ordinal	66th (sixty-sixth)
Factorization
Divisors	2, 3, 6, 11, 22, 33
Roman numeral	LXVI
Binary	1000010
Hexadecimal	42

In mathematics

66 is a sphenic number, a triangular number, a hexagonal number, and a semi-meandric number.

In science

• The atomic number of dysprosium, a lanthanide

In astronomy,

Messier object M66, a magnitude 10.0 galaxy in the constellation Leo

The New General Catalogue object NGC 66 , a peculiar barred spiral galaxy in the constellation Cetus

The Saros number of the solar eclipse series which began on -756 March 12 and ended on 542 May 1. The duration of Saros series 66 was 1298.1 years, and it contained 73 solar eclipses.

The Saros number of the lunar eclipse series which began on -671 August 12 and ended on 826 January 27. The duration of Saros series 66 was 1496.5 years, and it contained 84 lunar eclipses.

In other fields

66 is also:

• The designation of the historic Route 66, dubbed the "Mother Road" by novelist John Steinbeck.
• The designation of US Interstate 66, a freeway that runs from Virginia to Washington, D.C..
• The number of the French department Pyrénées-Orientales.
• The international direct dialling (IDD) code for Thailand.
• The year AD 66, 66 BC, or 1966.
• The name of a German card game, translated from sechsundsechzig.
• Sexual slang for anal sex.
• Number of the Hockey legend Mario Lemieux