Kilo, mega, giga, tera, peta, and all that

Prefix multipliers are used in data storage, data communications, electronics, and physics to denote extremely small and large quantities. Read definitions for terms including kilo, mega, giga, tera and peta.

Also see Kibi, mebi, gibi, tebi, pebi, and all that, which are relatively new prefixes designed to express power-of-two multiples.

Kilo, mega, giga, tera, and peta are among the list of prefixes that are used to denote the quantity of something, such as, in computing and telecommunications, a byte or a bit. Sometimes called prefix multipliers, these prefixes are also used in electronics and physics. Each multiplier consists of a one-letter abbreviation and the prefix that it stands for.

In communications, electronics, and physics, multipliers are defined in powers of 10 from 10-24 to 1024, proceeding in increments of three orders of magnitude (103 or 1,000). In IT and data storage, multipliers are defined in powers of 2 from 210 to 280, proceeding in increments of ten orders of magnitude (210 or 1,024). These multipliers are denoted in the following table.

 

Prefix Symbol(s) Power of 10 Power of 2
yocto- y 10-24 * --
zepto- z 10-21 * --
atto- a 10-18 * --
femto- f 10-15 * --
pico- p 10-12 * --
nano- n 10-9 * --
micro- m 10-6 * --
milli- m 10-3 * --
centi- c 10-2 * --
deci- d 10-1 * --
(none) -- 100 20
deka- D 101 * --
hecto- h 102 * --
kilo- k or K ** 103 210
mega- M 106 220
giga- G 109 230
tera- T 1012 240
peta- P 1015 250
exa- E 1018 * 260
zetta- Z 1021 * 270
yotta- Y 1024 * 280
* Not generally used to express data speed
** k = 103 and K = 210

Examples of quantities or phenomena in which power-of-10 prefix multipliers apply include frequency (including computer clock speeds), physical mass, power, energy, electrical voltage, and electrical current. Power-of-10 multipliers are also used to define binary data speeds. Thus, for example, 1 kbps (one kilobit per second) is equal to 103, or 1,000, bps (bits per second); 1 Mbps (one megabit per second) is equal to 106, or 1,000,000, bps. (The lowercase k is the technically correct symbol for kilo- when it represents 103, although the uppercase K is often used instead.)

When binary data is stored in memory or fixed media such as a hard drive, diskette, ZIP disk, tape, or CD-ROM, power-of-2 multipliers are used. Technically, the uppercase K should be used for kilo- when it represents 210. Therefore 1 KB (one kilobyte) is 210, or 1,024, bytes; 1 MB (one megabyte) is 220, or 1,048,576 bytes.

The choice of power-of-10 versus power-of-2 prefix multipliers can appear arbitrary. It helps to remember that in common usage, multiples of bits are almost always expressed in powers of 10, while multiples of bytes are almost always expressed in powers of 2. Rarely is data speed expressed in bytes per second, and rarely is data storage or memory expressed in bits. Such usages are considered improper. Confusion is not likely, therefore, provided one adheres strictly to the standard usages of the terms bit and byte.

This was first published in April 2008

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