the sum of Euler's totient function φ(x) over the first sixteen integers.

a semiperfect number, since adding up some subsets of its divisors (e.g., 1, 4, 5, 10, 20 and 40) gives 80.

a ménage number.

palindromic in bases 3 (2222_{3}), 6 (212_{6}), 9 (88_{9}), 15 (55_{15}), 19 (44_{19}) and 39 (22_{39}).

a repdigit in bases 3, 9, 15, 19 and 39.

a Harshad number in bases 2, 3, 4, 5, 6, 7, 9, 10, 11, 13, 15 and 16

The Pareto principle (also known as the 80-20 rule) states that, for many events, roughly 80% of the effects come from 20% of the causes.

Every solvable configuration of the Fifteen puzzle can be solved in no more than 80 single-tile moves.

In science

The atomic number of mercury

In religion

According to Exodus 7:7, Moses was 80 years old when he initially spoke to Pharaoh on behalf of his people. Today, 80 years of age is the upper age limit for cardinals to vote in papal elections.

In sports

Jerry Rice wore the number 80 for the majority of his NFL career. The number 80 is also the retired number of Seattle Seahawks wide receiver Steve Largent; when Rice was traded to the Seahawks, Largent gave the team permission to unretire his number so Rice could wear it.

In other fields

Eighty is also:

used in the classic book title Around the World in Eighty Days

the length of the Eighty Years' War or Dutch revolt (1568–1648)

the standard TCP/IP port number for HTTP connections

the 80A, 80B and 80C photographic filters correct for excessive redness under tungsten lighting

The year AD 80, 80 BC, or 1980

Eighty shilling ale

I-80 is an interstate highway going from New York to San Francisco

The older four-pin-base version of the 5Y3GT rectifier tube

A common limit for the characters per line, in computing, derived from the number of columns in IBM cards