Number

28

28 is a composite number, even, a calendar year.

Happy Number Hexagonal Keith Number Odious Number Padovan Number Perfect Number Pernicious Number Practical Number Recamán's Sequence Semiperfect Number Triangular Year

Historical context — 28 AD

Calendar year

AD 28 (XXVIII) was a leap year starting on Thursday of the Julian calendar.

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Historical context — 28 BC

Calendar year

Year 28 BC was either a common year starting on Saturday, Sunday or Monday or a leap year starting on Saturday or Sunday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar.

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Year facts

Year type: Leap year
Divisible by 4 and not by 100; February has 29 days.
Days in year: 366
ISO weeks: 52
Started on: Saturday
January 1, 28
Ended on: Sunday
December 31, 28
Friday the 13ths: 1
One Friday the 13th this year.
Decade: 20s
20–29
Century: 1st century
1–100
Millennium: 1st millennium
1–1000
Years ago: 1,998
1998 years before 2026.

In other calendars

Hebrew: 3788 / 3789 AM
Rosh Hashanah falls in September/October.
Chinese: Year of the zodiac:Earth zodiac:Rat
Sexagenary cycle position 25 of 60. Lunar new year falls in late January / mid-February.
Buddhist Era: 571 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
Ethiopian: 20 / 21 ET
Year boundary at Enkutatash (September 11/12).
Indian National (Saka): -50 / -51 Saka
Indian national calendar; year starts in March.

Properties

Parity: Even
Digit count: 2
Digit sum: 10
Digit product: 16
Digital root: 1
Palindrome: No
Bit width: 5 bits
Reversed: 82
Recamán's sequence: a(60) = 28
Square (n²): 784
Cube (n³): 21,952
Divisor count: 6
σ(n) — sum of divisors: 56
φ(n) — Euler's totient: 12
Sum of prime factors: 11

Primality

Prime factorization: 2 ² × 7

Nearest primes: 23 (−5) · 29 (+1)

Divisors & multiples

All divisors (6)

1 · 2 · 4 · 7 · 14 (half) · 28

Aliquot sum (sum of proper divisors): 28

Factor pairs (a × b = 28)

1 × 28

2 × 14

4 × 7

First multiples

28 · 56 (double) · 84 · 112 · 140 · 168 · 196 · 224 · 252 · 280

Sums & aliquot sequence

As a sum of two cubes: 1³ + 3³

As consecutive integers: 1 + 2 + … + 7

Aliquot sequence: 28 → 28 — reaches a perfect number

Representations

In words: twenty-eight
Ordinal: 28th
Roman numeral: XXVIII
Binary: 11100
Octal: 34
Hexadecimal: 0x1C
Base64: HA==
One's complement: 227 (8-bit)
Scientific notation: 2.8 × 10¹

In other bases

ternary (3) 1001

quaternary (4) 130

quinary (5) 103

senary (6) 44

septenary (7) 40

nonary (9) 31

undecimal (11) 26

duodecimal (12) 24

tridecimal (13) 22

tetradecimal (14) 20

pentadecimal (15) 1d

Palindromic in base 3

Historical numeral systems

Babylonian (base 60): 𒌋𒌋𒁹𒁹𒁹𒁹𒁹𒁹𒁹𒁹
Egyptian hieroglyphic: 𓎆𓎆𓏺𓏺𓏺𓏺𓏺𓏺𓏺𓏺
Greek (Milesian): κηʹ
Mayan (base 20): 𝋡·𝋨
Chinese: 二十八
Chinese (financial): 貳拾捌

In other modern scripts

Eastern Arabic ٢٨ Devanagari २८ Bengali ২৮ Tamil ௨௮ Thai ๒๘ Tibetan ༢༨ Khmer ២៨ Lao ໒໘ Burmese ၂၈

Digit at this position in famous constants

π — Pi (π): Digit 28 = 3
e — Euler's number (e): Digit 28 = 1
φ — Golden ratio (φ): Digit 28 = 4
√2 — Pythagoras's (√2): Digit 28 = 4
ln 2 — Natural log of 2: Digit 28 = 1
γ — Euler-Mascheroni (γ): Digit 28 = 0

Also seen as

Goldbach decomposition

Goldbach's conjecture says every even integer greater than 2 is the sum of two primes. For 28, here are decompositions:

5 + 23 = 28
11 + 17 = 28

ASCII character

As an ASCII codepoint, 28 is control character (0x1C). ASCII control character.

Hex color

#00001C

RGB(0, 0, 28)

IPv4 address

As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.28.

Address: 0.0.0.28
Class: reserved
IPv4-mapped IPv6: ::ffff:0.0.0.28

Unspecified address (0.0.0.0/8) — "this network" placeholder.