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For other uses, see Giga (disambiguation).

Giga (/ˈɪɡə/ or /ˈɡɪɡə/) is a unit prefix in the metric system denoting a factor of a (short-form) billion (109 or 1000000000). It has the symbol G.

Giga is derived from the Greek word γίγας, 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 Conference in 1947: "The following prefixes to abbreviations for the names of units should be used: G giga 109×".

When referring to information units in computing, such as gigabyte, giga may sometimes mean 1073741824 (230), although such use is inconsistent, contrary to standards and has been discouraged by the standards organizations.[1][2] The inconsistency is that gigabit is never (or very rarely) used with the binary interpretation of the prefix, while gigabyte is sometimes used this way. The binary prefix gibi has been adopted for 230, while reserving giga exclusively for the metric definition.


In English, the initial g of giga can be pronounced /ɡ/ (a hard g as in giggle), or /ˈɪɡə/ (a soft g as in giant, like a j sound, which shares its Greek root).[3]

This latter pronunciation was formalized within the United States in the 1960s and 1980s with the issue by the US National Bureau of Standards of pronunciation guides for the metric prefixes.[4] A prominent example is found in the pronunciation of gigawatts in the 1985 movie 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 by the humorous poet Christian Morgenstern that appeared in the third (1908) edition of Galgenlieder (Gallows Songs). This suggests that a hard German [ɡ] was originally intended as the pronunciation. Self was unable to ascertain at what point the alternative pronunciation came into occasional use, but claimed that as of 1995 it had died out.[5]

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

Common usage[edit]

Prefix 1000m 10n Decimal English word Adoption[nb 1]
Name Symbol Short scale Long scale
yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  10007  1021 1000000000000000000000  sextillion  thousand trillion or trilliard 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  thousand billion or billiard 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  thousand million or milliard 1960
mega M  10002  106 1000000             million 1960 (1873)
kilo k  10001  103 1000             thousand 1960 (1795)
hecto h  10002/3  102 100             hundred 1960 (1795)
deca da  10001/3  101 10             ten 1960 (1795)
 10000  100 1             one
deci d  1000−1/3  10−1 0.1             tenth 1960 (1795)
centi c  1000−2/3   10−2 0.01             hundredth 1960 (1795)
milli m  1000−1  10−3 0.001             thousandth 1960 (1795)
micro μ  1000−2  10−6 0.000001             millionth 1960 (1873)
nano n  1000−3  10−9 0.000000001  billionth  thousand millionth 1960
pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960
femto f  1000−5  10−15 0.000000000000001  quadrillionth  thousand billionth 1964
atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  thousand trillionth 1991
yocto y  1000−8  10−24  0.000000000000000000000001  septillionth  quadrillionth  1991
  1. ^ The metric system was introduced in 1795 with several metric prefixes, of which, however, only six were adopted as SI prefixes by the 11th CGPM conference in 1960, whereas myria (104) as well as double and demi were not adopted. In 1873, micro and mega were recommended by the British Association for the Advancement of Science. The other dates relate to recognition by a resolution of the CGPM.

International Electrotechnical Commission (IEC) standard[edit]

As a unit of information, 1 GB incorrectly represents 1,073,741,824 or 230 bytes. In IEC standard, the correct notation of 1,073,741,824 is gibi (abbreviated to Gi). So, 1GiB is 1,073,741,824 bytes ( 1 billion and 73 million 741 thousand 824 bytes ) or 1.074GB (when rounded up). A laptop wrongly advertised as having 8GB has indeed 8,589,934,592 bytes of memory or 8.59GB.

See also[edit]


  1. ^ "§3.1 SI prefixes". The International System of Units (SI) (PDF) (in French and English) (8th ed.). Paris: STEDI Media. 2006. p. 127. ISBN 92-822-2213-6. Retrieved 2007-02-25. [Side note:] These SI prefixes refer strictly to powers of 10. They should not be used to indicate powers of 2 (for example, one kilobit represents 1000 bits and not 1024 bits). The IEC has adopted prefixes for binary powers in the international standard IEC 60027-2: 2005, third edition, Letter symbols to be used in electrical technology — Part 2: Telecommunications and electronics. The names and symbols for the prefixes corresponding to 210, 220, 230, 240, 250, and 260 are, respectively: kibi, Ki; mebi, Mi; gibi, Gi; tebi, Ti; pebi, Pi; and exbi, Ei. Thus, for example, one kibibyte would be written: 1 KiB = 210 B = 1024 B, where B denotes a byte. Although these prefixes are not part of the SI, they should be used in the field of information technology to avoid the incorrect usage of the SI prefixes. 
  2. ^ NIST Guide for the Use of the International System of Units (Appendix D. ref 5)
  3. ^ A Practical Guide to the International System of Units, U.S. Metric Association, Feb 2008 Archived June 13, 2010, at the Wayback Machine.
  4. ^ NBS Special Publication 304 & 304A, revised August 1981, "A Brief History of Measurement Systems"
  5. ^ Kevin Self, April 1995, "Technically speaking", Spectrum
  6. ^ Wells, J.C. (1998). LPD pronunciation preference poll 1998.

External links[edit]