Prefix Multipliers

Kilo, mega, giga, tera, and peta are prefixes 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-

µ

10-6 *

--

milli-

m

10-3 *

--

centi-

c

10-2 *

--

deci-

d

10-1 *

--

(none)

--

100

20

1*

deka-

D

10

--

hecto-

h

102 *

--

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

21 *

70

kilo-

Examples of quantities or phenomena in which power-of10 prefix multipliers apply include frequency (including computer clock speeds), physical mass, power, energy, electrical voltage, and electrical current. Power-of-10 multipiers 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.

Kibi, mebi, gibi, tebi, pebi, and exbi are binary prefix zettaZ 10 2 multipliers that, in 1998, were yottaY 1024 * 280 approved as a standard by the International Electrotechnical * Not generally used to express data speed Commission (IEC) in an effort to eliminate the confusion that ** k = 103 and K = 210 sometimes occurs between decimal (power-of-10) and binary (power-of-2) numeration terms.

At present, the prefix multipliers kilo- (k or K), mega- (M), giga- (G), tera- (T), peta- (P), and exa- (E) are ambiguous. In most of the physical sciences, and when describing quantities of objects generally, these multipliers refer to powers of 10. However, when used to define data quantity in terms of bytes, they refer to powers of 2. The following table denotes the most often-used prefixes and their meanings. Symbol(s)

Power of 10 Quantities of x

Power of 2 Bytes of Data

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

Prefix kilo-

The power-of-10 multipliers and the power-of-2 multipliers for a given word prefix are almost, but not quite, the same. For example, the power-of-10 definition of kilo- (k) refers to 1,000, while the powerof-2 definition (K) refers to 1,024. As if this is not confusing enough, when referring to a data speed of one kilobit per second (1 kbps), analysts generally mean 1,000 bits per second (103 bps), but when talking about one kilobyte (1 KB) of data storage, they usually mean 1,024 bytes (210 B). This prevailing confusion could be eliminated (some computer scientists believe) by adopting special prefixes referring to the binary quantities. The proposed scheme is as follows.

* Not generally used to express data speed ** k = 103 and K = 210

Full technical Proposed name prefix

Proposed symbol

Numeric multiplier

kilobinary

kibi-

Ki

210

megabinary

mebi-

Mi

220

gigabinary

gibi-

Gi

230 40

terabinary

tebi-

Ti

2

petabinary

pebi-

Pi

250

In scenarios such as the one mentioned above, if the new binary prefixes are used, it should be easy to know whether an engineer is talking or writing about the decimal or binary multiplier. We will know that one kilobit per second (1 kbps) means 1,000 bps, and one kibibyte (1 KiB) means 1,024 bytes, for example. As of this writing, the binary prefix multipliers have not yet come into general use.

Pronunciation: Based on a suggestion from NIST, "the first syllable of the name of the binary-multiple prefix exabinary exbiEi 260 should be pronounced in the same way as the first syllable of the name of the corresponding International Standard (SI) prefix, and the second syllable should be pronounced as 'bee.'" Thus, "kibi" would be pronounced "KIH-bee"; "mebi" would be "MEH-bee", and so forth.