How to determine the maximum RAM capacity for an operating system?

I was curious to know what limits the max RAM capacity for an OS while reading about microprocessors being 32-bit and 64-bit. I know that limit for 32-bit OS is 4GB and for 64-bit OS is 16 Exabytes, but my question is how do we get there? I found this calculation:

$\qquad 2^{32} = 4\,294\,967\,296$

and

$\qquad \frac{4\,294\,967\,296}{1024 \cdot 1024}\,\mathrm{B} = 4\,096\, \mathrm{MB} = 4\, \mathrm{GB}$.

It's different for 64-bit:

$\qquad 2^{64} = 18\,446\,744\,073\,709\,551\,616$

and

$\qquad \frac{18\,446\,744\,073\,709\,551\,616}{1024 \cdot 1024}\mathrm{B} = 16\,\mathrm{EB}$.

What I don't understand is how the calculation of bits turned into bytes and reached the results 4GB and 16EXB?

The calculation is in bytes since the memory is addressed in units of bytes. If a machine word is $N$ bits and an addressed is stored in a register whose size is a single machine word, then you can address $2^N$ different locations, each of which is a byte (in principle, that could depend on the CPU, but I'm not aware of any CPU which addresses its memory in larger words). However, more complicated addressing mechanisms can be used to address more memory. For example, pointers could take up more than a machine word, or segments could be applied (as in 80386 and its successors).

StackExchange Computer Science Q#19619, answer score: 5