18 (number)

← 17 19 → 18
← 10 11 12 13 14 15 16 17 18 19 → List of numbers — Integers 0 10 20 30 40 50 60 70 80 90 →
Cardinal	eighteen
Ordinal	18th (eighteenth)
Numeral system	octodecimal
Factorization	$2 \cdot 3^2$
Divisors	1, 2, 3, 6, 9, 18
Roman numeral	XVIII
Binary	100102
Octal	228
Duodecimal	1612
Hexadecimal	1216

18 (eighteen) is the natural number following 17 and preceding 19.

In speech, the numbers 18 and 80 are sometimes confused. When carefully enunciated, they differ in which syllable is stressed: 18 /eɪtˈtiːn/ vs 80 /ˈeɪti/. However, in dates such as 1864, or when contrasting numbers in the teens, such as 17, 18, 19, the stress shifts to the first syllable: 18 /ˈeɪttiːn/. In some dialects, such as General American, there is little confusion because the single t sound in 80 becomes a d-like flap [ɾ], whereas the double t sound in 18 does not.

In mathematics

Eighteen is a composite number, its proper divisors being 1, 2, 3, 6 and 9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a semiperfect number. Eighteen is the first inverted square-prime of the form p.q².

It is a heptagonal number, and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number. It is also the sum of the totient function for the first seven integers.

Eighteen is the second abundant number with the aliquot sum of 21 (117% in abundance) in the aliquot sequence (18, 21, 11, 1, 0) thus being the first composite number in the 11-aliquot tree. Eighteen is the aliquot sum of only one number the square 289.

In base 10 it is a Harshad number.

18, aside from 0, is the only number that equals twice the sum of its decimal digits.

In science

• The atomic number of argon
• The 18-electron rule is a rule of thumb in transition metal chemistry for characterising and predicting the stability of metal complexes
• The number of years in the Saros cycle of eclipses of the sun and moon

In religion and literature

• The Hebrew word for "life" is חי (chai), which has a numerical value of 18. Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life.
• In Judaism, in the Talmud; Pirkei Avot (5:25), Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married ("Ben shmonah esra lechupah", at eighteen years old to the Chupah (marriage canopy)). (See Coming of age, Age of majority).
• In Ancient Roman custom the number 18 can symbolise a blood relative.
• Joseph Heller's novel Catch-22 was originally named Catch-18 because of the Hebrew meaning of the number, but was amended to the published title to avoid confusion with another war novel, Mila 18.
• There are 18 chapters in the Bhagavad Gita, which is contained in the Mahabharata, which has 18 books. The Kurukshetra War which the epic depicts, is between 18 armies (11 on the Kuru side, 7 on the Pandava).
• A shape looking like 18 in Arabic numbers is formed by the lines on the palm of the right hand.

As lucky or unlucky number

• In Chinese tradition, the number 18 is normally 十八 (shí bā), but it can also be read as 幺八 (yāo bā), which sounds like 要发 (yào fā), meaning that one is going to prosper. Thus, building floors numbered "18" are often very expensive in China.^{[citation needed]} Contrastingly, though, the floor 18 of a building in Northern China is viewed as a floor to be avoided, as one can never return from the 18th level of hell.^{[citation needed]}

Age 18

In most countries 18 is the age of majority.
In most countries 18 is the voting age.
In many countries 18 is the age of sexual consent. 18 is the age when a person can watch an NC-17 movie.

See also: 18 certificate, Coming of age, and Voting age

In other fields

Eighteen is also:

• 18:00, a time on the 24-hour clock corresponding to 6:00 PM.
• A 2005 movie directed by Richard Bell and starring Brendan Fletcher and Alan Cumming.
• The number of a car in the NASCAR Sprint Cup Series currently owned by Joe Gibbs Racing. The #18 has been driven by Kyle Busch since 2008. Other notable drivers of the car include Dale Jarrett, who won the 1993 Daytona 500 in the #18; Bobby Labonte, who won the Cup championship in 2000; and J. J. Yeley.
• A 2002 album by electronic artist Moby.
• The number of holes in a stipulated round of golf
• The number of chapters into which James Joyce's epic novel Ulysses is divided
• Russ Wheeler's car number in the film Days of Thunder
• A slang term used in association football for the Penalty Area
• The number of wheels on the most common type of North American tractor-trailer truck, which are hence often called 18-wheelers
• The number of the French department Cher and the Turkish province Bitlis
• DVD-18 is double-sided, double-layered DVD format
• A Canon error message, see E18 error
• In neo-Nazi circles, a code word for Adolf Hitler. The number comes from the position of the letters in the alphabet: A = 1, H = 8. See also 88.
• Android 18, a fictional character in the metaseries Dragon Ball.
• 18th Street gang, a Hispanic American gang
• The Hindu epic Mahabharata has eighteen sections, involves eighteen armies and is about a war fought over eighteen days
• In some countries the number 18 means blood (relative)
• In the Chinese mythos, Hell has 18 levels
• In Chinese folklore, the Shaolin temple had a group of 18 bronze Monks. Initiates could only be considered full graduates of Shaolin martial arts if they could defeat them in combat.
• In 1990 Megadeth released Hanger 18 as a single for the Rust in Piece Album.
• Eighteen Visions (often abbreviated to 18V), an alternative metal/metalcore band from Orange County, California.
• Eighteen Eighteen, a South African Rap artist.
• "I'm Eighteen", Alice Cooper's first Top Ten hit single, from their 1971 album Love It to Death.
• Eighteen is also a surname, recorded in East Anglia in the 18th century

Historical years

• A.D. 18
• 18 B.C.
• 1918
• 2018

See also

• Numeral system