Number reference
18 explained.
18: eighteen, composite; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- eighteen
- Classification
- composite
- Factors
- 1, 2, 3, 6, 9, 18
- Prime factorization
- 2 × 3 × 3
- Square root
- 4.242641
- Cube root
- 2.620741
- Binary
- 10010
- Octal
- 22
- Hexadecimal
- 12
- Roman numeral
- XVIII
- Digit sum
- 9
- Digital root
- 9
What makes 18 notable
- Factor structure. 18 breaks down into 2 distinct primes (2, 3) and has 6 divisors in total, so it can be split into equal whole-number groups 4 ways beyond 1 and itself.
18 in everyday use
Eighteen is the age of legal adulthood in most countries, bringing the vote, contracts, and military service. It is the atomic number of argon, the gas used in incandescent bulbs and welding. A full round of golf is eighteen holes, a convention fixed at St Andrews in 1764, and a large articulated lorry in the United States is an eighteen-wheeler. In Hebrew, the letters that spell "chai", meaning life, add up to 18, so gifts and donations in Jewish communities are often given in multiples of 18.
Eighteen is the only number that is exactly twice the sum of its digits: 1 + 8 = 9 and 9 × 2 = 18. It factors as 2 × 3², giving six divisors, and it is abundant, since 1 + 2 + 3 + 6 + 9 = 21 exceeds it. It is also the number of degrees between neighbouring points on a twenty-pointed star, since 360 divided by 20 is 18.
Nearby numbers
How to read this page
The factor list shows every way 18 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 18 for contexts that do not use decimal.
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