SI prefix

<unit, standard>

The standard metric prefixes used in the Système International d'Unités (SI) conventions for scientific measurement.

Here are the SI magnifying prefixes, along with the corresponding binary interpretations in common use: 

 prefix abr decimal  binary

 yocto-     1000^-8
 zepto-     1000^-7
 atto-      1000^-6
 femto-  f  1000^-5
 pico-   p  1000^-4
 nano-   n  1000^-3
 micro-  *  1000^-2          * Abbreviation: Greek mu
 milli-  m  1000^-1

 kilo- 	 k  1000^1  1024^1 = 2^10 = 1,024
 mega- 	 M  1000^2  1024^2 = 2^20 = 1,048,576
 giga- 	 G  1000^3  1024^3 = 2^30 = 1,073,741,824
 tera- 	 T  1000^4  1024^4 = 2^40 = 1,099,511,627,776
 peta- 	    1000^5  1024^5 = 2^50 = 1,125,899,906,842,624
 exa-  	    1000^6  1024^6 = 2^60 = 1,152,921,504,606,846,976
 zetta-     1000^7  1024^7 = 2^70 = 1,180,591,620,717,411,303,424
 yotta-     1000^8  1024^8 = 2^80 = 1,208,925,819,614,629,174,706,176
"Femto" and "atto" derive not from Greek but from Danish.

The abbreviated forms of these prefixes are common in electronics and physics.

When used with bytes of storage, these prefixes usually denote multiplication by powers of 1024 = 2^10 (K, M, G and T are common in computing). Thus "MB" stands for megabytes (2^20 bytes). This common practice goes against the edicts of the BIPM who deprecate the use of these prefixes for powers of two. The formal SI prefix for 1000 is lower case "k"; some, including this dictionary, use this strictly, reserving upper case "K" for multiplication by 1024 (KB is thus "kilobytes").

Also, in data transfer rates the prefixes stand for powers of ten so, for example, 28.8 kb/s means 28,800 bits per second.

The unit is often dropped so one may talk of "a 40K salary" (40000 dollars) or "2 meg of disk space" (2*2^20 bytes).

The accepted pronunciation of the initial G of "giga-" is hard, /gi'ga/.

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. For example, 3.5" microfloppies are often described as storing "1.44 MB". In fact, this is completely specious. The correct size is 1440 KB = 1440 * 1024 = 1474560 bytes. Alas, this point is probably lost on the world forever.

In 1993, hacker Morgan Burke proposed, to general approval on Usenet, the following additional prefixes: groucho (10^-30), harpo (10^-27), harpi (10^27), grouchi (10^30). This would leave the prefixes zeppo-, gummo-, and chico- available for future expansion. Sadly, there is little immediate prospect that Mr. Burke's eminently sensible proposal will be ratified.

Last updated: 2009-09-01

Nearby terms:

siod ♦ SIP ♦ SIPB ♦ SIPLAN ♦ SIPP ♦ SI prefix ♦ Siprol ♦ SIR ♦ SIRDS ♦ Siri ♦ SIRTS

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