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

Twenty-eight is a composite number and the second perfect number as it is the sum of its proper divisors: ${\displaystyle 1+2+4+7+14=28}$. As a perfect number, it is related to the Mersenne prime 7, since ${\displaystyle 2^{3-1}\times (2^{3}-1)=28}$. The next perfect number is 496, the previous being 6.

Though perfect, 28 is not the aliquot sum of any other number other than itself; thus, it is not part of a multi-number aliquot sequence.

Twenty-eight is the sum of the totient function for the first nine integers.

Since the greatest prime factor of ${\displaystyle 28^{2}+1=785}$ is 157, which is more than 28 twice, 28 is a Størmer number.

Twenty-eight is a harmonic divisor number, a happy number, the 7th triangular number, a hexagonal number, a Leyland number of the second kind (${\displaystyle 2^{6}-6^{2}}$), and a centered nonagonal number.

It appears in the Padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these).

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