In techspeak and jargon, the standard metric prefixes used in the SI (Système international) conventions for scientific measurement have dual uses. With units of time or things that come in powers of 10, such as money, they retain their usual meanings of multiplication by powers of 1000 = 103. But when used with bytes or other things that naturally come in powers of 2, they usually denote multiplication by powers of 1024 = 210. Here are the magnifying prefixes in jargon use:
prefix decimal binary
kilo- 10001 10241 = 210 = 1,024
mega- 10002 10242 = 220 = 1,048,576
giga- 10003 10243 = 230 = 1,073,741,824
tera- 10004 10244 = 240 = 1,099,511,627,776
peta- 10005 10245 = 250 = 1,125,899,906,842,624
exa- 10006 10246 = 260 = 1,152,921,504,606,846,976
Here are the fractional prefixes:
*prefix decimal jargon usage*
milli- 1000-1 (seldom used in jargon)
micro- 1000-2 small or human-scale (see micro-)
nano- 1000-3 even smaller (see nano-)
pico- 1000-4 even smaller yet (see pico-)
femto- 1000-5 (not used in jargon--yet)
atto- 1000-6 (not used in jargon--yet)
The binary peta- and exa- loadings are not in common use--yet, and the prefix milli-, denoting multiplication by 1000-1, has always been rare (there is, however, a standard joke about the 'millihelen' -- notionally, the amount of beauty required to launch one ship).
'Femto' and 'atto' (which, interestingly, derive not from Greek but from Danish) have not yet acquired jargon loadings, though it is easy to predict what those will be once computing technology enters the required realms of magnitude (however, see attoparsec).
There are, of course, some standard unit prefixes for powers of 10. In the following table, the 'prefix' column is the international standard suffix for the appropriate power of ten; the 'binary' column lists jargon abbreviations and words for the corresponding power of
2. The B-suffixed forms are commonly used for byte quantities; the words 'meg' and 'gig' are nouns which may (but do not always) pluralize with 's'.
prefix decimal binary pronunciation
kilo- k K, KB, /kay/
mega- M M, MB, meg /meg/
giga- G G, GB, gig /gig/,/jig/
Confusingly, hackers often use K as though it were a suffix or numeric multiplier rather than a prefix; thus "2K dollars". This is also true (though less commonly) of G and M.
Note that the formal SI metric prefix for 1000 is 'k'; some use this strictly, reserving 'K' for multiplication by 1024 (KB is 'kilobytes').
K, M, and G used alone refer to quantities of bytes; thus, 64G is 64 gigabytes and 'a K' is a kilobyte (compare mainstream use of 'a G' as short for 'a grand', that is, $1000). Whether one pronounces 'gig' with hard or soft 'g' depends on what one thinks the proper pronunciation of 'giga-' is.
Confusing 1000 and 1024 (or other powers of 2 and 10 close in magnitude) -- for example, describing a memory in units of 500K or 524K instead of 512K -- is a sure sign of the marketroid.