Binary vs. Decimal Capacity Measurements

Computer measurements are expressed in both binary and decimal terms, often using the same notation. Due to a mathematical coincidence, the fact that 2^10 (1024) is almost the same number as 10^3 (1000), there are two similar but different ways to express a megabyte or a gigabyte.

The problems with binary and decimal are probably more noticed in the area of hard disk capacity than anywhere else. Hard disk manufacturers always use binary figures for their products' capacity: a 640GB hard disk has about 640,000,000,000 bytes of storage. However, hard disk makers also use binary numbers where they are normally used--for example, buffer capacities are expressed in binary kilobytes or megabytes--but the same notation ("kB" or "MB") is used as for decimal figures. Hard disks are large, and larger numbers cause the discrepancy between decimal and binary terms to be exaggerated. For example, a 1TB hard disk, expressed in binary terms, is "only" 931GB. Since most software uses binary terms, this difference in numbers is the source of frequent confusion regarding "where the rest of the gigabytes went". In fact, they didn't go anywhere. It's just a different way of expressing the same thing.

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