If you’ve ever purchased a computer with a hard disk capacity of 500 GB and opened Windows Explorer only to find that its capacity looked more like 440 GB, you may be wondering where all those gigabytes went.

There are several reasons Windows could display the wrong amount of available space, from invisible shadow files, formatting overhead, and hidden recovery partitions to misleading (though technically accurate) storage capacities advertised by hard drive manufacturers.
Why Your Hard Drive Shows Less Space Than Advertised

If you’ve paid attention to hard drives, USB flash drives, and other storage devices, you may have noticed that they always have less space than promised once they’re formatted. The reason for this difference lies in the way hard drive manufacturers advertise their devices, versus the way Windows computers actually use the storage devices. There’s also some overhead required when Windows formats your drive, for the file system and boot data, though in comparison to today’s large hard drives, it’s not a lot.

To a hard disk manufacturer, one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB. Essentially, if a hard disk is advertised as 500GB, it contains 500 * 1000 * 1000 * 1000 = 500,000,000,000 bytes of space. The hard disk manufacturer thus advertises the disk as a 500 GB hard disk.

However, manufacturers of RAM don’t sell it in even groups of 1000 – they use groups of 1024. When you’re buying memory, a KB is 1024 bytes, a MB is 1024 KB, and a GB is 1024 MB. To work back from the 500,000,000,000 bytes

Keep in mind that the hard drive manufacturers are using the accurate description of the terms–the prefix giga, for instance, means a power of 1000, whereas the correct term for powers of 1024 is gibibyte, though it isn’t often used. Unfortunately, Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000.


That’s a difference of nearly 35 GB over what the average buyer would be led to believe a hard drive contains. If hard disks were advertised in terms of the amount of space they actually contained when you connected them to your Windows computer, a 1 TB hard drive would be labeled a 931 GB hard drive instead.

Alternatively, Windows could update their UI to use the correct definition of gigabyte–other operating systems, like OS X, have already changed their representation to correctly state the right amount of space.

http://www.howtogeek.com/123268/windows-hard-drive-wrong-capacity/

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This is all true.

I was expecting an explaination of disk memory space that's used by hidden partitions, file allocation tables, etc