Twenty-eight is the sum of the first five prime numbers, as well as a triangular number and the second perfect number. The next perfect number is 496.

**Twenty-eight** is also

- The atomic number of nickel.
- The number of days in the shortest month of the Gregorian calendar, February. (Except on 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).

This article is about the number. For the year AD 28, see 28.