Unit prefix

[1] In addition to those listed in the everyday-use table, the SI includes standardised prefixes for 1015 (peta), 1018 (exa), 1021 (zetta), 1024 (yotta), 1027 (ronna), and 1030 (quetta); and for 10−15 (femto), 10−18 (atto), 10−21 (zepto), 10−24 (yocto), 10−27 (ronto), and 10−30 (quecto).

For example, a 500-gigabyte hard drive holds 500 billion bytes, and a 100-megabit-per-second Ethernet connection transfers data at 100 million bits per second.

Before the adoption of ronna and quetta for 1027 and 1030 and ronto and quecto for 10−27 and 10−30 in November 2022, many personal, and sometimes facetious, proposals for additional metric prefixes were formulated.

[14][15][16][17][18] In 2010, an online petition sought to establish hella- as the SI prefix for 1027, a movement that began on the campus of UC Davis.

[19][20] The prefix, which has since appeared in the San Francisco Chronicle, Daily Telegraph, Wired and some other scientific magazines, was recognised by Google, in a non-serious fashion, in May 2010.

They were changed because of previously proposed ascending hepto (Greek "hepta" (7)) was already in use as a numerical prefix (implying seven) and the letter "h" as both SI-accepted non-SI unit (hour) and prefix (hecto 102), the same applied to "s" from previously proposed descending septo (i.e. SI unit "s", seconds), while "o" for octo was problematic since a symbol "o" could be confused with zero.

Using Greek for ascending and Latin for descending would be consistent with established prefixes such as deca, hecto, kilo vs. deci, centi, milli.

[26] In 2001, a few unofficial prefixes appeared on the Internet: hepa (1021), ento (10−21), otta (1024), fito (10−24), nea (1027), syto (10−27), dea (1030), tredo (10−30), una (1033) and revo (10−33).

The Oxford professor Jeffrey K. Aronson has suggested extending beyond zetta/zepto and yotta/yocto with xenta/xenno, wekta/weko, vendeka/vendeko, and udeka/udeko, based on the idea that the "Z" and "Y" prefixes would continue backwards through the English alphabet.

He goes on to list a large number of prefixes, starting with Xona, Weka, Vunda, Uda, Treda, Sorta, ... Another proposal for xenta/xona is novetta, from the Italian "nove" (or "nine").

In 1993, Morgan Burke proposed, as a joke, harpo for 10−27, groucho for 10−30 (and therefore harpi for 1027, grouchi for 1030, zeppi for 1033, gummi for 1036, and chici for 1039), all of these being references to the comedy act, the Marx Brothers.