Prefixes for very high numbers

Many English words for units of measurement are formed with the help of prefixes derived from the Greek and Latin words for particular numbers - for example, millisecond, centimetre, kilogram. (See Prefixes in units of measurement). However, the words for the very large and very small units of measurement which have become necessary with the development of modern science and technology and with the advent of computers and IT do not make use of the Greek or Latin words for particular numbers.

This is because the Greeks and Romans did not have single word equivalents for very large numbers such as a million, let alone a billion, or a trillion. The Greeks did have a word for 10,000 - murioi: it is the root of the English word 'myriad' - but they had no single word for any number higher than this, and in fact murioi was often used not to mean 10,000 precisely but simply 'numberless, countless, or measureless'. The Romans had a word for 1000 (mille) but no single word for any number larger than this, and very large numbers had to be expressed as multiples of 1000. So a million in Latin is decies centum milia, i.e., ten times a hundred thousand.

Nonetheless, the English words for very large or very small units of measurement do make use of prefixes which derive from Greek words: mega-, nano-, and tera-, for example, all have a Greek origin. (Incidentally, the prefix kilo- (as in 'kilowatt' and 'kilobyte') does derive from a Greek word for a specific number: it comes from the word chilioi, which means a thousand. A kilowatt is 1000 watts, and a kilobyte is close to 1000 bytes, it is in fact 1024 bytes, i.e., 2 raised to the 10th power. See Prefixes in units of measurement.)

The following table gives the details. Note that, as a general rule, the abbreviations for numbers larger than 1 are written with upper case (capital) letters; those for numbers smaller than 1 are written with lower case (small) letters - but prefixes dealing with smaller multiples above 1, deca- ('ten times'), hecto- (one hundred') and kilo- ('one thousand'), which were first used in 1795 with the introduction of the metric system, are always used with lower case letters.

Greek word and meaning	Prefix	Meaning of prefix	abbreviated form as prefix	Examples
megas - 'big', 'great'	mega-	1,000,000 - a million (i.e., 10 to the 6th power) OR (in computing) 1,048, 576 (i.e. 2 to the 20th power)	M	megawatt, megahertz, megavolt, megaton, megabit, megabyte, megaflop
gigas - 'giant', 'mighty'	giga-	1,000,000,000 - a billion (i.e., a thousand million, 10 to the 9th power); OR (in computing) 2 to the 30th power	G	gigavolt, gigahertz, gigabyte, gigaflop
teras - 'marvel', 'portent', 'monster'	tera-	1,000,000,000,000 - a trillion (i.e., a million million, 10 to the 12th power); OR (in computing) 2 to the 40th power	T	terametre, terabyte, teraflop
See below	peta-	1,000,000,000,000,000 - a thousand trillion (i.e., a thousand million million, 10 to the 15th power; OR (in computing) 2 to the 50th power	P	petametre, petabyte, petaflop
milli - ' thousand (Latin)	milli-	1/1000 - one-thousandth (10−3)	m	milligram, millisecond
mikros - 'small'	micro-	1/1,000,000 - a millionth (1×10−6)	μ Gk. letter mu	microsecond, micrometre

Don't confuse milligram (mg), with the Latin m- prefix, and 'microgram (μg), with the Greek μ- prefix
      This has caused serious problems within medical treatments.

nanos - 'dwarf'	nano-	1/1,000,000,000 - a billionth (1×10−9)	n	nanosecond, nanometre
[Spanish] pico- - 'a bit'	pico-	1/1,000,000,000,000 - a trillionth (1×10−12)	p	picovolt, picokelvin, picocurie The prefix peta- does not derive from any Greek word, but was coined on the model of the prefix tera- to designate the next unit in the Système International d'Unités: it seems that the sequence of prefixes tera-/peta- in this series was felt to parallel the sequence of prefixes tetra-/penta- in such series as tetrahedron/pentahedron or tetrameter/pentameter. (For the meaning of the prefixes tetra- and penta- see Prefixes in words for geometrical figures).