27 (twenty-seven; Roman numeral XXVII) is the natural number following 26 and preceding 28.

Twenty-seven is the cube of 3, or three tetrated ${\displaystyle ^{2}3=3^{3}=3\times 3\times 3}$, divisible by the number of prime numbers below it (nine).

The first non-trivial decagonal number is 27.^[1]

27 has an aliquot sum of 13^[2] (the sixth prime number) in the aliquot sequence ${\displaystyle (27,13,1,0)}$ of only one composite number, rooted in the 13-aliquot tree.^[3]

The sum of the first four composite numbers is ${\displaystyle 4+6+8+9=27}$,^[4] while the sum of the first four prime numbers is ${\displaystyle 2+3+5+7=17}$,^[5] with 7 the fourth indexed prime.^{[6][lower-alpha 1]}

In the Collatz conjecture (i.e. the ${\displaystyle 3n+1}$ problem), a starting value of 27 requires 3 × 37 = 111 steps to reach 1, more than any smaller number.^{[10][lower-alpha 2]}

27 is also the fourth perfect totient number — as are all powers of 3 — with its adjacent members 15 and 39 adding to twice 27.^{[13][lower-alpha 3]}

A prime reciprocal magic square based on multiples of ${\displaystyle {\tfrac {1}{7}}}$ in a ${\displaystyle 6\times 6}$ square has a magic constant of 27.

Including the null-motif, there are 27 distinct hypergraph motifs.^[14]

There are exactly twenty-seven straight lines on a smooth cubic surface,^[15] which give a basis of the fundamental representation of Lie algebra ${\displaystyle \mathrm {E_{6}} }$.^[16][17]

The unique simple formally real Jordan algebra, the exceptional Jordan algebra of self-adjoint 3 by 3 matrices of quaternions, is 27-dimensional;^[18] its automorphism group is the 52-dimensional exceptional Lie algebra ${\displaystyle \mathrm {F_{4}} .}$^[19]

There are twenty-seven sporadic groups, if the non-strict group of Lie type ${\displaystyle \mathrm {T} }$ (with an irreducible representation that is twice that of ${\displaystyle \mathrm {F_{4}} }$ in 104 dimensions)^[20] is included.^[21]

In Robin's theorem for the Riemann hypothesis, twenty-seven integers fail to hold ${\displaystyle \sigma (n)<e^{\gamma }n\log \log n}$ for values ${\displaystyle n\leq 5040,}$ where ${\displaystyle \gamma }$ is the Euler–Mascheroni constant; this hypothesis is true if and only if this inequality holds for every larger ${\displaystyle n.}$^[22][23][24]

Base-specific

In decimal, 27 is the first composite number not divisible by any of its digits, as well as:

• the third Smith number^[25] and sixteenth Harshad number,^[26]
• the only positive integer that is three times the sum of its digits,
• equal to the sum of the numbers between and including its digits: ${\displaystyle 2+3+4+5+6+7=27}$.

Also in base ten, if one cyclically rotates the digits of a three-digit number that is a multiple of 27, the new number is also a multiple of 27. For example, 378, 783, and 837 are all divisible by 27.

• In similar fashion, any multiple of 27 can be mirrored and spaced with a zero each for another multiple of 27 (i.e. 27 and 702, 54 and 405, and 378 and 80703 are all multiples of 27).
• Any multiple of 27 with "000" or "999" inserted yields another multiple of 27 (20007, 29997, 50004, and 59994 are all multiples of 27).

In senary (base six), one can readily test for divisibility by 43 (decimal 27) by seeing if the last three digits of the number match 000, 043, 130, 213, 300, 343, 430, or 513.

In decimal representation, 27 is located at the twenty-eighth (and twenty-ninth) digit after the decimal point in π:

$${\displaystyle 3.141\;592\;653\;589\;793\;238\;462\;643\;383\;{\color {red}27}9\ldots }$$

If one starts counting with zero, 27 is the second self-locating string after 6, of only a few known.^[27][28]

• The Moon revolves around the Earth the same direction as Earth spins but 27 (27.3) times slower.
• 27km is the circumference of the LHC located at CERN in Meyrin Switzerland
• The atomic number of cobalt.
• Dark matter is thought to make up 27% of the universe.^[29]
• 27 is the number of bones in the human hand.^[30]

• The number of letters in the Spanish alphabet.^[33]
• The number of books in the New Testament.
• The number of generations from David to the Christ according to the Gospel of Matthew. (Mt 1,1-17)
• The number of Grievances of the United States Declaration of Independence.
• The number of ratified Amendments to the United States Constitution.
• Until 1835, the English Alphabet consisted of 27 letters: after "Z" the 27th letter of the alphabet was Ampersand (&)
• The total number of letters in the Hebrew alphabet (22 regular letters and 5 final consonants).
• Alternate name for The Hunt, a book by William Diehl.
• Abbé Faria's prisoner number in the book The Count of Monte Cristo.
• In Stephen King's novel It, It returns every 27 years to Derry.
• The war led by the Antichrist will begin in the year 1999 and will last 27 years according to Nostradamus.

• 27 Nakṣatra or lunar mansions in Hindu astrology.

• The value of all the colors in snooker add up to 27.
• The number of outs in a regulation baseball game for each team at all adult levels, including professional play, is 27.
• The New York Yankees have won 27 World Series championships, the most of any team in the MLB.

Twenty-seven is also:

• A-27, American attack aircraft.
• The code for international direct-dial phone calls to South Africa.
• The name of a cigarette, Marlboro Blend No. 27.
• The number of the French department Eure.
• The current number of countries in the European Union, as of 2024.

• List of highways numbered 27
• List of highways numbered 27A