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I bet this happened with every one of you. You buy a brand new 100GB hard disk and happly after installation checks system manager to painfully realize that its shows 93GB in total. You get confused! Did the vendor cheated me?? or is it for some funky system usage they have reserved up my precious 7 GB?

You are not alone! The fact we finds out the hard way is that there are two ways to define a gigabyte!

When you buy a “100 Gigabyte” hard drive, the vendor defines it using the decimal powers of ten definition of the “Giga” prefix.

100 * 10⁹ bytes = 100,000,000,000 = 100 Gigabytes

But the operating system determines the size of the drive using the computer’s binary powers of two definition of the “Giga” prefix:

93 *  2³⁰  bytes = 99,857,989,632 = 93 Gigabytes

If you’re wondering where 7 Gigabytes of your 100 Gigabyte drive just disappeared to, you have the answers. It’s an old trick by hard drive makers– they intentionally use the official SI definitions of the Giga prefix so they can inflate the sizes of their hard drives, at least on paper. This was always an annoyance, but now it’s much more difficult to ignore, as it results in large discrepancies with today’s enormous hard drives. When your Terabyte hard drive is not a Terabyte? Its 931 GB.