28
XXVIII
11100
The number 28 — factors and divisors
Perfect
28 = 2² × 7. It has 6 divisors, adding up to 56.
Its divisors, itself excluded, add up to exactly the number.
Families it belongs to
Triangular
The running total of 1, 2, 3 … — the dots stack into a triangle.
Perfect
Add up its divisors except itself and you get the number back.
Laid out as dots
It fits as 4 × 7. The more divisors, the closer to a square.
- Prime factors
- 2² × 7
- Divisors
- 1, 2, 4, 7, 14, 28
- Sum of divisors
- 56
- Sum without itself
- 28
- Coprime below it
- 12
- Digit sum
- 10
- Digital root
- 1
- Roman numeral
- XXVIII
- Binary
- 11100
- Octal
- 34
- Hexadecimal
- 1C
- Base 36
- S
- Bits
- 5 bits
- Collatz steps
- 18 steps · peaks at 52
- Prime before
- 23
- Prime after
- 29
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 28 a prime number?
No. It breaks down into 2² × 7.
Q. What are the divisors of 28?
1, 2, 4, 7, 14, 28 — 6 divisors in all, adding up to 56.
Q. How is 28 written in binary?
11100, which is 5 bits. In hexadecimal it is 1C.
Q. How many Collatz steps does 28 take?
It reaches 1 in 18, peaking at 52 on the way.