18
XVIII
10010
The number 18 — factors and divisors
Abundant
18 = 2 × 3². It has 6 divisors, adding up to 39.
Its divisors, itself excluded, add up to more than the number.
Laid out as dots
It fits as 3 × 6. The more divisors, the closer to a square.
- Prime factors
- 2 × 3²
- Divisors
- 1, 2, 3, 6, 9, 18
- Sum of divisors
- 39
- Sum without itself
- 21
- Coprime below it
- 6
- Digit sum
- 9
- Digital root
- 9
- Roman numeral
- XVIII
- Binary
- 10010
- Octal
- 22
- Hexadecimal
- 12
- Base 36
- I
- Bits
- 5 bits
- Collatz steps
- 20 steps · peaks at 52
- Prime before
- 17
- Prime after
- 19
The grid
Ten per row, twenty rows. The filled cells are primes — you can see the multiples of 2 and 5 drop out in columns.
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Numbers either side
How to read this
- The prime factors are what the number is made of. The count and sum of divisors both follow from them.
- Add the divisors, take the number away, and you get perfect, abundant or deficient.
- The length of the binary form is the bit count. 255 taking eight digits is what a byte is.
- Collatz: halve it if even, triple it and add one if odd. The count is how many moves reach 1.
Frequently asked questions
Q. Is 18 a prime number?
No. It breaks down into 2 × 3².
Q. What are the divisors of 18?
1, 2, 3, 6, 9, 18 — 6 divisors in all, adding up to 39.
Q. How is 18 written in binary?
10010, which is 5 bits. In hexadecimal it is 12.
Q. How many Collatz steps does 18 take?
It reaches 1 in 20, peaking at 52 on the way.