- 28 is the second perfect number.
- 28 is an even composite number.
- 28 is the 7th triangular number, and therefore the number of tiles in a double-six domino set.
- 28 is equal to the sum of its proper divisors: 1 + 2 + 4 + 7 + 14 = 28. This makes it a perfect number, the neighboring ones being 6 and 496.
- Ignoring Easter, the Julian calendar repeats every 28 years.
In Greek-based number-naming systems, 28 is associated with prefix "octoicosa-", and with prefix "octoviginti-" in Latin systems.
- MathWorld, Perfect Number