The number 18
18 is a composite number with 6 divisors. Its prime factorisation is 2 × . Divisors (6): 1, 2, 3, 6, 9, 18.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 18: a Harshad number: divisible by its digit sum 9.
- 2
18 is composite. Its prime factorisation is
- 3
The divisors add up to 39; without 18 itself that is 21, so 18 is abundant (21 is more than 18). Euler's totient φ(18) = 6 numbers up to 18 share no factor with it.
- 4
The digits of 18 add up to 9, and repeating that until one digit is left gives the digital root 9. The Collatz sequence (halve if even, else 3n+1) takes 20 steps to reach 1.
LaTeX source of the formulas on this page
\[ 18 = 2 \times 3^{2} \]18 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 6 |
| Sum of divisors | 39 |
| Euler totient φ(n) | 6 |
| Square | 324 |
| Cube | 5832 |
| Square root | 4.242641 (= 3√2) |
| Digit sum / digital root | 9 / 9 |
| Collatz steps to 1 | 20 |
| Scientific notation | 1.8 × |
18 in other forms
| Form | Value |
|---|---|
| In words | eighteen |
| Roman numerals | XVIII |
| Binary (base 2) | 10010 |
| Octal (base 8) | 22 |
| Hexadecimal (base 16) | 12 |
| Base 3 | 200 |
| As time | 18 seconds |
Is 18 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | Yes (18 ÷ 2 = 9) | A number is divisible by 2 if the last digit is even (0, 2, 4, 6, 8). |
| 3 | Yes (18 ÷ 3 = 6) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | No (remainder 2) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | No (remainder 3) | A number is divisible by 5 if the last digit is 0 or 5. |
| 6 | Yes (18 ÷ 6 = 3) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 4) | A number is divisible by 7 if double the last digit and subtract it from the rest: the result is a multiple of 7. |
| 8 | No (remainder 2) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | Yes (18 ÷ 9 = 2) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 8) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 7) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 6) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 18 a prime number?
No. 18 = 2 × , so it is divisible by 2.
What are the factors of 18?
1, 2, 3, 6, 9, 18.
What is 18 in binary?
10010 (hexadecimal 12, octal 22).
What is the square root of 18?
About 4.2426, exactly 3√2.
What is 18 in Roman numerals?
XVIII.
Is 18 divisible by 3?
Yes. Its digits add up to 9, a multiple of 3.
What is special about the number 18?
18 is a Harshad number: divisible by its digit sum 9.