128
CXXVIII
10000000
The number 128 — factors and divisors
Deficient
128 = 2⁷. It has 8 divisors, adding up to 255.
Its divisors, itself excluded, add up to less than the number.
Families it belongs to
Power of two
Two multiplied by itself. Every extra bit doubles it.
Laid out as dots
It fits as 8 × 16. The more divisors, the closer to a square.
- Prime factors
- 2⁷
- Divisors
- 1, 2, 4, 8, 16, 32, 64, 128
- Sum of divisors
- 255
- Sum without itself
- 127
- Coprime below it
- 64
- Digit sum
- 11
- Digital root
- 2
- Roman numeral
- CXXVIII
- Binary
- 10000000
- Octal
- 200
- Hexadecimal
- 80
- Base 36
- 3K
- Bits
- 8 bits
- Collatz steps
- 7 steps · peaks at 128
- Prime before
- 127
- Prime after
- 131
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 128 a prime number?
No. It breaks down into 2⁷.
Q. What are the divisors of 128?
1, 2, 4, 8, 16, 32, 64, 128 — 8 divisors in all, adding up to 255.
Q. How is 128 written in binary?
10000000, which is 8 bits. In hexadecimal it is 80.
Q. How many Collatz steps does 128 take?
It reaches 1 in 7, peaking at 128 on the way.