28 (twenty-eight) is the natural number following 27 and preceding 29.

Cardinal	twenty-eight
Ordinal	twenty-eighth
Factorization
Roman numeral	XXVIII
Binary	11100
Hexadecimal	1C

In mathematics

It is a composite number, its proper divisors being 1, 2, 4, 7 and 14, making it a perfect number and a harmonic divisor number.

Twenty-eight is the second perfect number. As a perfect number, it is related to the Mersenne prime 7, since 2²(2³ - 1) = 28. The next perfect number is 496.

Twenty-eight is a triangular number, a hexagonal number and a centered nonagonal number. Twenty-eight is the sum of the first five prime numbers.

It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 2, 8, 10, 18, 28...

Twenty-eight is the ninth and last number in early Indian Magic square of order 3.

In science

• The atomic number of nickel
• The fourth Magic number (physics)

In astronomy,

Messier object M28, a magnitude 8.5 globular cluster in the constellation Sagittarius

The New General Catalogue object NGC 28 , an elliptical galaxy in the constellation Phoenix

The Saros number of the solar eclipse series which began on -1910 March 22 and ended on -630 April 28. The duration of Saros series 28 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -1915 June 27 and ended on -581 September 6. The duration of Saros series 28 was 1334.2 years, and it contained 75 lunar eclipses.

In other fields

Twenty-eight is:

• The number of days in the shortest month of the Gregorian calendar, February. (Except in leap years, when there are twenty-nine.)
• The common name for the parrot Barnardius zonarius semitorquatus, widely distributed in Western Australia and South Australia. Its call sounds like "wenniate".
• The number of letters in the Danish and Swedish alphabets.
• In neo-Nazi circles, twenty-eight indicates Blood and Honour (28 = BH - B - second letter of the alphabet and H - the eight letter).
• Part of the title of a romantic comedy movie 28 Days , and an unrelated science fiction movie 28 Days Later.
• Part of the name of a well known indie rock band from Melbourne, Australia called 28 Days - the name is unrelated to either of the movies above.
• Historical years: 28 A.D., 28 B.C., or 1928
• The number of Chinese constellations, "Xiu" or "mansions" (a literal translation), equivalent to the 12 western zodiac constellations.
• The number of normal human teeth, not including the third molars (wisdom teeth).
• The number of dominoes in standard domino sets.