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SI prefixes and their etymologies

SI prefixes

PrefixSymbolFactorOrdinary Notation*Name
yottaY10241 000 000 000 000 000 000 000 000septillion**
zettaZ10211 000 000 000 000 000 000 000sextillion**
exaE10181 000 000 000 000 000 000quintillion**
petaP10151 000 000 000 000 000quadrillion**
teraT10121 000 000 000 000trillion**
gigaG1091 000 000 000billion**
megaM1061 000 000million
kilok1031 000thousand
 
hectoh102100hundred
dekada10110ten
 
1001one
 
decid10-10.1tenth
centic10-20.01hundredth
 
millim10-30.001thousandth
microμ10-60.000 001millionth
nanon10-90.000 000 001billionth**
picop10-120.000 000 000 001trillionth**
femtof10-150.000 000 000 000 001quadrillionth**
attoa10-180.000 000 000 000 000 001quintillionth**
zeptoz10-210.000 000 000 000 000 000 001sextillionth**
yoctoy10-240.000 000 000 000 000 000 000 001septillionth**
*The “ordinary notation” column uses spaces to separate numbers into groups of three digits. In the US, commas are more often used for this purpose, but spaces have the advantage of avoiding confusion when numbers might be seen by readers who use the comma as the decimal point.
**The terms billion, trillion, etc., can be ambiguous. The terms are used here in the American English sense, but British English traditionally defined billion as a million million, rather than a thousand million. However, recent British usage tends to match the American meanings.

SI prefix etymologies

PrefixPronunciationOrigin
yottaYOTE-uh or YOTT-uhLatin: “eight” [(103)8]
zettaZETT-uhLatin: “seven” [(103)7]
exaEX-uhGreek: “six” [(103)6]
petaPET-uhGreek: “five” [(103)5]
teraTAIR-uhGreek: “monster”
gigaJIG-uh or GIG-uhGreek: “giant”
megaMEG-uhGreek: “big”
kiloKILL-ohGreek: “thousand”
hectoHECK-toeGreek: “hundred”
dekaDECK-uhGreek: “ten”
 
deciDESS-ihLatin: “tenth”
centiSENT-ihLatin: “hundredth
milliMILL-ihLatin: “thousandth”
microMIKE-roeGreek: “small”
nanoNAN-ohGreek: “dwarf”
picoPEEK-ohSpanish: “tiny bit”
femtoFEM-toeDano-Norwegian: “fifteen”
attoAT-toeDano-Norwegian: “eighteen”
zeptoZEP-toeLatin: “seven” [(103)-7]
yoctoYOCK-toeLatin: “eight” [(103)-8]

These tables were provided by David Bernklau, Certified Metrication Specialist (CMS), The first table is an expanded version of Table 2 published in Practical Guide to the International System of Units by Dennis Brownridge.

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Updated: 2014-03-21