Giga (/ˈɡɪɡə/ or /ˈɪɡə/) 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×."[1]

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.[2][3] 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 /ˈɡɪɡə/ (a hard g as in giggle), or /ˈɪɡə/ (a soft g as in giant, which shares giga's Greek root).[4]

This latter pronunciation was formalised 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.[5] A prominent example 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).[6][7] This suggests that a hard German [ɡ] was originally intended as the pronunciation. Self was unable to ascertain when the /dʒ/ (soft g) pronunciation came into occasional use, but claimed that as of 1995 it had returned to /ɡ/ (hard g).[8][9]

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).[10]

Common usage

Prefix Base 1000 Base 10 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  trilliard 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  billiard 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  milliard 1960
mega M  10002  106 1000000  million 1873
kilo k  10001  103 1000  thousand 1795
hecto h  10002/3  102 100  hundred 1795
deca da  10001/3  101 10  ten 1795
 10000  100 1  one
deci d  1000−1/3  10−1 0.1  tenth 1795
centi c  1000−2/3   10−2 0.01  hundredth 1795
milli m  1000−1  10−3 0.001  thousandth 1795
micro μ  1000−2  10−6 0.000001  millionth 1873
nano n  1000−3  10−9 0.000000001  billionth  milliardth 1960
pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960
femto f  1000−5  10−15 0.000000000000001  quadrillionth  billiardth 1964
atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  trilliardth 1991
yocto y  1000−8  10−24  0.000000000000000000000001  septillionth  quadrillionth 1991
  1. Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.

Binary prefix

The notation 1 GB can represent 1,073,741,824 (230) bytes or 1,000,000,000 bytes. Under the IEC 60027-2 A.2 and ISO/IEC 80000 standards, the correct notation of 230 is gibi (symbol Gi). So, 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: 8.59×109 B, or 8 GiB.

See also


  1. "giga-, comb. form". Oxford English Dictionary. October 2011.
  2. "§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.
  3. NIST Guide for the Use of the International System of Units (Appendix D. ref 5)
  4. "SI prefixes and their etymologies". US Metric Association. Retrieved 27 November 2019.
  5. NBS Special Publication 304 & 304A, revised August 1981, "A Brief History of Measurement Systems"
  6. Morgenstern, Christian (1917). Galgenlieder nebst dem 'Gingganz' (in German). Illustrated by Karl Walser (22 ed.). Berlin, Germany: Bruno Cassirer. p. 52 via Project Gutenberg. [First four lines:] Im Anfang lebte, wie bekannt, / als größter Säuger der Gig-ant. / Wobei gig eine Zahl ist, die / es nicht mehr gibt, - so groß war sie! [These lines are the only appearance of gig in the book. Gigant is German for "giant"; cf. "gigantic".]
  7. Morgenstern, Christian (1963). Gallows Songs: Christian Morgenstern's "Galgenlieder", Bilingual Edition: A Selection. Translated by Knight, Max. University of California Press. pp. 24–25. ISBN 9780520008847. Retrieved 20 February 2016. [Translation:] Of yore, on earth was dominant / the biggest mammal: the Gig-ant. / ("Gig" is a numeral so vast, / it's been extinct for ages past.)
  8. Self, Kevin (October 1994). "Technically speaking". Spectrum. IEEE: 18.
  9. Self, Kevin (April 1995). "Technically speaking". Spectrum. IEEE: 16.
  10. Wells, J. C. (1998). LPD pronunciation preference poll 1998.
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