Giga-

Giga- ( or ) is a unit prefix in the metric system denoting a factor of a short-scale billion or long-scale milliard (109 or 1,000,000,000). It has the symbol G.

Giga- is derived from the Greek word γίγας (gígas), meaning "giant". The Oxford English Dictionary reports the earliest written use of giga in this sense to be in the Reports of the IUPAC 14th Conférence Internationale de Chimie in 1947: "The following prefixes to abbreviations for the names of units should be used: G giga 109×." However, it was already used in 1932 by the German organization Verband deutscher Elektrotechniker.

When referring to information units in computing, such as gigabyte, giga may sometimes mean 1 073  741  824 (230); this causes ambiguity. Standards organizations discourage this and use giga- to refer to 109 in this context too. Gigabit is only rarely used with the binary interpretation of the prefix. The binary prefix gibi has been adopted for 230, while reserving giga exclusively for the metric definition.

Pronunciation
In English, the prefix giga can be pronounced (a hard g as in giggle), or  (a soft g as in gigantic, which shares giga Ancient Greek root). A prominent example of this latter pronunciation is found in the pronunciation of gigawatts in the 1985 film Back to the Future.

According to the American writer Kevin Self, a German committee member of the International Electrotechnical Commission proposed giga as a prefix for 109 in the 1920s, drawing on a verse (evidently "Anto-logie") by the German humorous poet Christian Morgenstern that appeared in the third (1908) edition of his Galgenlieder (Gallows Songs). This suggests that a hard German was originally intended as the pronunciation. Self was unable to ascertain when the (soft g) pronunciation came into occasional use, but claimed that as of 1995 it had returned to  (hard g).

In 1998, a poll by the phonetician John C. Wells found that 84% of Britons preferred the pronunciation of gigabyte starting with (as in gig), 9% with  (as in jig), 6% with  (guy), and 1% with  (as in giant).

Common usage

 * gigahertz—clock rate of a CPU, for instance, 3 GHz = 3 000  000  000  Hz
 * gigabit—bandwidth of a network link, for instance, 1 Gbit/s = 1 000  000  000  bit/s.
 * gigabyte—for instance, for hard disk capacity, 120 GB = 120 000  000  000  bytes;
 * gigayear or gigaannum—one billion (109) years, sometimes abbreviated Gyr, but the preferred usage is Ga or, for years ago, GA.

Binary prefix
The notation $1 GB$ represents 1,000,000,000 bytes or, in deprecated usage, 1,073,741,824 (230) bytes. Per IEC 60027-2 A.2 and ISO/IEC 80000 standards, the correct notation of 230 is gibi (symbol Gi). One gibibyte ($1 GiB$) is 1,073,741,824 bytes or $1.074 GB$. Despite international standards, the use of $1 GB$ = 230 B is widespread. A laptop advertised as having $8 GB$ has 8,589,934,592 bytes of memory: $8.59 B$, or $8 GiB$.