Just as an example of this problem, you may have a 320Gb capacity hard disk drive installed in your computer, but your OS (Operating System) such as Windows, only shows there's 298Gb of space available. So why the discrepancy?
The reason is that computers have a different way of describing how much storage space is available. For example, 1K bytes is not actually 1000 bytes, but 1024 bytes. However, as humans we like nice round figures to keep things simple, even if not strictly correct.
If we divide 320Gb by 1.024, we come up with a figure of 312Gb (instead of 298Gb); however, this is obviously incorrect and for a good reason.As there are 1024 bytes in 1Kb, and then 1024 Kbs in 1 Mb, and yet again 1024Mbs in 1Gb, the figure we need to use for the division is 1.024 x 1.024 x 1.024 - which is 1.073.
you can divide our example hard drive size of 320Gb by 1.073 and we successfully get 298Gb of available storage, which is what your operating system is likely to show.
The magic figure to divide our hard drive in Gbs is therefore 1.073.
Do try it on some other hard disk drive sizes and see if it works for you?