28
28 is the second perfect number: it equals the sum of its proper divisors (1+2+4+7+14).
- Even
- Composite
Where 28 fits
Why 28 is special
28 connects a clean mathematical property (the second perfect number, known since antiquity) to the closest whole-number approximation of the lunar month, which underlies February's day count.
Not just a mathematical number
28is also used in ways where ordinary arithmetic doesn't apply — each listed below with what genuinely does and doesn't follow from it.
- Mathematical number
A genuine quantity — its mathematical properties (prime, square, factors, and so on) are real facts about it.
A genuine quantity that ordinary arithmetic applies to — it can be prime, even, a perfect square, added to another number meaningfully, and so on.
- Quantity
Also used as a quantity — Adult human skeleton.
A genuine count or amount of something — closest to a pure mathematical number, but tied to what's being counted (12 eggs is a genuine amount of eggs, not an abstract mathematical fact about the number 12).
28 in Numbers Around Us
Factors & digit properties
- Divisors (6)
- 1, 2, 4, 7, 14, 28
- Prime factorisation
- 2^2 × 7
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 28
- Binary
- 11100
- Octal
- 34
- Hexadecimal
- 1c
- Roman numeral
- XXVIII
- Scientific notation
- 2.800 × 10^1
Other mathematical patterns
- Triangular number
- Perfect number
- Happy number
- Hexagonal number
Related numbers & connections
14 is a factor of 28 (14 × 2 = 28).
View 14 →A typical adult mouth has 28 teeth, excluding third molars (wisdom teeth).
Science
The Moon's orbital (synodic) period is approximately 29.5 days; 28 is a commonly cited rounded approximation of a lunar month, and February's 28-day (29 in leap years) length is often loosely associated with it.
DocumentedScienceThe Moon's orbital (synodic) period is approximately 29.5 days; 28 is a commonly cited rounded approximation of a lunar month, and February's 28-day (29 in leap years) length is often loosely associated with it.
DocumentedScience
Common questions
Is 28 a prime number?
No, 28 is not prime.
What are the factors of 28?
The factors of 28 are 1, 2, 4, 7, 14, 28.
What is 28 in Roman numerals?
28 in Roman numerals is XXVIII.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026