Number reference
28 explained.
28: twenty-eight, composite, perfect number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- twenty-eight
- Classification
- composite
- Factors
- 1, 2, 4, 7, 14, 28
- Prime factorization
- 2 × 2 × 7
- Square root
- 5.291503
- Cube root
- 3.036589
- Binary
- 11100
- Octal
- 34
- Hexadecimal
- 1C
- Roman numeral
- XXVIII
- Digit sum
- 10
- Digital root
- 1
What makes 28 notable
- Perfect number. 28 is a perfect number — its proper divisors (1 + 2 + 4 + 7 + 14) sum to exactly 28. Only three perfect numbers exist below one thousand.
- Triangular number. 28 is the 7th triangular number: it is the sum 1 + 2 + … + 7, the count of items in a triangle with 7 rows.
28 in everyday use
Twenty-eight days make February in a common year and the average menstrual cycle, and 28 dominoes make a double-six set. It is the atomic number of nickel. An ounce is a little over 28 grams, and the lunar month runs a bit over 28 days at just under 29.5. Twenty-eight is also the standard number of teeth in an adult mouth before wisdom teeth arrive.
Twenty-eight is the second perfect number: its proper divisors 1, 2, 4, 7, and 14 add up to exactly 28. Only 6 comes before it, and 496 is next. It is the seventh triangular number and the sum of the first five primes, 2 + 3 + 5 + 7 + 11. Every known perfect number is triangular, and 28 shows why: it is 7 × 8 ÷ 2.
Nearby numbers
How to read this page
The factor list shows every way 28 splits into equal whole-number groups — 6 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 28 for contexts that do not use decimal.
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