Hard Disk Drive (HDD) are an integral part of any personal computers. Their primary role is to store data. The amount of data that can be stored depends on the capacity. If you did not notice, you will discover that our beloved operating system: Micosoft Windows, always give a flawed report of the amount of HDD space you actually have. The worst thing is that it’s always lesser than what you have. This is partly their fault. But they are right for the other part.

Units of Count
Manufacturers uses the widely known (by you and me) Megabyte (MB), Gigabyte (GB), etc. This is the International System of Units (SI) unit. However, the more accurate representation of data capacity is the lesser known Mebibyte (MiB) system. The MiB system is also known as the International Electrotechnical Commission (IEC) binary prefix, it is built around base 2, which differs from the more commercially used MB that is base 10.

The Truth
The kibibyte is 2¹⁰ bytes (1024 bytes); the mebibyte is 2²⁰ bytes (1024 kilobytes); the gibibyte 2³⁰ bytes and so on.

For example, you happily bought a new 100GB HDD, which is 100,000,000,000 bytes or 93.13GiB. Incidently, 93,13GiB is the number that Windows reports back to you, but incorrectly labelling it as 93.13GB.

Conclusion
It is the non-standard way in representing data capacity. It is not the manufacturers’ fault and it is not entirely Window’s fault, they do get things done the right way sometimes.

Reference: Atomic MPC