28

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, the previous being 6.

The revolution time of the surface of the Sun on itself is 28 days while its core is revolving in 33 days.

In Gematriya, the system of Hebrew Numerology, the number 28 corresponds to the word koakh, meaning "power", "energy".

The number of dominoes in standard domino sets.

The postal code of the province of Madrid, in Spain.