18 (number)
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18 is a natural number that comes after 17 and before 19. It is an even number, meaning it divides evenly into two equal parts of 9 each. It is written as '18' in decimal notation, while in Roman numerals it is XVIII.
18 is a composite number with divisors 1, 2, 3, 6, 9, and 18. Its prime factorization is 2 × 3 × 3, or 2 × 3². The sum of its proper divisors is 1 + 2 + 3 + 6 + 9 = 21, which exceeds 18; therefore 18 is classified as an abundant number.
Mathematical properties
18 is two times 9 and three times 6. It is divisible by 1, 2, 3, 6, and 9. It is also a Harshad number in base 10: the sum of its digits (1 + 8 = 9) divides 18. When written in binary it is 10010.
In geometry, 18 appears in calculations involving the roots of shapes. For example, the square root of 18 is not a whole number; the square root of 18 is approximately 4.24. Additionally, 18 is the number of sides of an 18-sided polygon (octadecagon).
Social and legal use
Many countries have designated 18 as the age of legal majority, which affects voting rights, contracts, and criminal responsibility. Legal systems vary across countries, so 18 is not a uniform standard worldwide.
In mathematics and number systems, 18 appears in various contexts: it is twice nine, and nine has significance in base-10 notation because of divisibility rules. Additionally, 18 belongs to numbers commonly used in time measurement and group division.