What is a Gigabyte?

Exactly how big is "One Gigabyte?  Surprisingly, you will get different answers from different people.  This stems from the conflicting use of the binary and decimal number systems.

Two Different Gigabytes

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The prefix "giga" can be applied to many units of measure, such as a gigawatt, or gigahertz, just like "kilo" can be used for kilowatts or kilohertz.  When used to describe such units, the terms "kilo", "mega", and "giga"  mean 1,000  1,000,000  and 1,000,000,000 of these units. But when applied to computer storage devices (memory and disk drives) the terms have traditionally been interpreted in the binary sense, simply because computers address storage using the binary, not decimal, number system. 

Computer storage, including disk drives, used to be specified in binary KB, MB, and GB. But a few years ago some disk drive manufacturers decided to change the way they rate their drives, and started using decimal bytes -- millions and billions -- rather than binary MB and GB.  Using decimal values makes their drives seem larger. For example, a drive that holds, say, 10,200,547,328 bytes is a 9.5 GB drive in binary terms, but a "10.2 gigabyte drive" in decimal terms.  Certainly a "10.2 GB drive" sounds larger than a "9.5 GB drive", and is often listed in a different size category ("over 10 GB" vs "under 10 GB").  The larger the drive, the greater the difference.  A 128 GB drive holds over 137 billion bytes, and most drive makers would now call it a "137 gigabyte drive".

However, the operating system, which operates in binary, reports the drive size in binary. So if you buy a disk drive that's advertised as a "40 GB drive", that probably means it has 40 billion (40,000,000,000) bytes of capacity.  Your computer, which counts in binary (base 2) notation, will report it as a 37.2 GB drive.  This leaves many people thinking that nearly 3 billion bytes were "lost", when in fact it's just the difference between counting in base 10 versus base 2, and that's what caused the confusion.

Resolving the Confusion

There have been some attempts to clarify whether a stated value is decimal or binary. The IEC (International Electrotechnical Commission) has proposed the use of GiB (a "gibibyte", which is the contraction of Giga-binary-Byte) to mean binary, and GB to mean decimal. Unfortunately this convention has not been widely adopted, and is not widely understood when it is used.

Until GiB comes into common use and the difference between GB and GiB is understood by the public, there will still be confusion about what a "GB" means. This confusion is most commonly over the size of disk drives and some tape drives, but other values such as file size may also be specified in binary or decimal values. Memory will always be referred to in binary KB, MB, or GB (KiB, MiB, or GiB).  A memory board containing 1,000,000 bytes (vs a true binary megabyte) simply wouldn't work, so no one will try to market a "megabyte RAM chip" with only one million bytes.

When it's not clearly stated, you should expect that computer memory (RAM) will be specified in binary GiB, disk drives will be in decimal GB, and tape drives will be specified both ways. Units other than computer storage, for example frequency, have always been specified in decimal values. A 2 GHz CPU is a 2,000,000,000 Hertz device.

Additional Information

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