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Metric (SI) Prefixes

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A benefit of the SI (International System of Units) is that written technical information is effectively communicated, transcending the variations of language – including spelling and pronunciation. Arabic numerals describe the quantity. A quantity is then paired with a unit symbol, often with a prefix symbol that modifies unit magnitude.

In the SI, designations of multiples and subdivision of any unit may be arrived at by combining with the name of the unit the prefixes deka, hecto, and kilo meaning, respectively, 10, 100, and 1000, and deci, centi, and milli, meaning, respectively, one-tenth, one-hundredth, and one-thousandth. In certain cases, particularly in scientific usage, it becomes convenient to provide for multiples larger than 1000 and for subdivisions smaller than one-thousandth. The following table of 20 SI prefixes ranging from 1024 to 10−24 are currently recognized for use.

Purpose Name Preferred Pronunciation Symbol Factor Name

larger quantities
or whole units
yotta Yä-tuh Y 1024 Septillion
zetta ZETT-uh Z 1021 Sextillion
exa EX-uh E 1018 Quintillion
peta PET-uh P 1015 Quadrillion
tera TAIR-uh
Example: terahertz
T 1012 Trillion
giga JIG-uh
Example: gigawatt
G 109 Billion
mega MEG-uh M 106 Million
kilo KILL-oh
Example: kiloliter
k 103 Thousand
hecto HECK-toe
Example: hectare
h 102 Hundred
deka DECK-uh
Example: dekameter
da 101 Ten
        100 One
smaller quantities
or sub units

deci DESS-ih
Example: decimeter
d 10-1 Tenth
centi SENT-ih
Example: centigram
c 10-2 Hundredth
milli MILL-ih
Example: milliliter
m 10-3 Thousandth
micro MI-crow
Example: microgram
μ 10-6 Millionth
nano NAN-oh
Example: nanometer
n 10-9 Billionth
pico PEEK-oh
Example: picogram
p 10-12 Trillionth
femto FEM-toe
Example: femtosecond
f 10-15 Quadrillionth
atto AT-toe a 10-18 Quintillionth
zepto ZEP-toe
Example: zeptosecond
z 10-21 Sextillionth
yocto YOCK-toe
Example: yoctosecond
y 10-24 Septillionth

The simplified table below shows common metric prefixes and the relationship with their place values. Note that the recommended decimal sign or marker for use in the United States is the dot on the line, which is used to separate whole numbers from parts. Use a leading zero for numbers less than one. The convention of writing a zero before the decimal point is used to ensure that the quantity is appropriately interpreted.

Whole Units Decimal Units
thousands hundreds tens SI unit* tenths hundredths thousandths
1000 100 10 1 0.1 0.01 0.001
kilo- hecto- deka- meter
deci- centi- milli

   * SI base or derived units with special names may be used

Prefix Progress. Since the Metric System was first developed there have been four (4) key prefix updates. This chronological summary highlights the interesting history of SI prefixes.    

  • 1795 – The original 8 SI prefixes that were officially adopted: deca, hecto, kilo, myria, deci, centi, milli, and myrio, derived from Greek and Latin numbers. Initially, all were represented by lowercase symbols.
  • 1866 – The U.S. Metric Act illustrates how some now obsolete prefixes were used to expressed units, such as myriameter.
  • 1889 – First General Conference on Weights and Measures (CGPM) approves the 8 prefixes for use.
  • 1960 – Two prefixes were made obsolete (myria and myrio) and 6 were added, including 3 for forming multiples (mega, giga, tera) and 3 for forming submultiples (micro, nano, pico). Total Prefixes: 12.
  • 1964 – Two prefixes for forming submultiples were added (femto and atto), creating a situation where there were more prefixes for small than large quantities. Total Prefixes: 14.
  • 1975 – Two prefixes for forming multiples were added (peta and exa). Total Prefixes: 16.
  • 1991 – Four prefixes were added. Two for forming multiples (zetta and yotta) and 2 for forming submultiples (zepto and yocto). Total Prefixes: 20.

Capitalization. SI prefixes for submultiples (smaller quantities or sub units) are formatted with all lowercase symbols while prefixes for multiples (larger quantities or whole units) use uppercase symbols with the exception of three: kilo (k), hecto (h) and deka (da). 

Historical Exception. For historical reasons, the name "kilogram" for the SI base unit of mass contains the name "kilo," the SI prefix for 103. Thus, because compound prefixes are unacceptable, symbols for decimal multiples and submultiples of the unit of mass are formed by attaching SI prefix symbols to g (gram). The names of such multiples and submultiples are formed by attaching SI prefix names to the name "gram." Example: 1 mg, NOT 1 μkg (1 microkilogram).

Spelling. It’s important to note that spelling in NIST publications are made in accordance with the United States Government Printing Office Style Manual, which follows American English writing practices found in Webster's Third New International Dictionary. For example, the prefix deka is used (American English spelling) but not deca (British English). Webster’s Third New International Dictionary provides written pronunciation guidance, which may be supplemented by the online audio pronunciation links available in the Prefix table (above).

Pronunciation. Guidance is provided to supplement limited information available in SI writing style guidance publications and to aid general public use of the metric system. Writing with Metric Units discusses common best practices for effectively using SI practices in written communications and is based on NIST LC LC 1137, Metric Style Guide for the News Media.

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Credit: Pixabay

FAQ: How do I pronounce the prefix giga? The classic pronunciation for the scientific term giga is jig'a (soft “g”). The hard "g" pronunciation of giga is also frequently heard in common parlance. Some Prefix Etymology resources list both soft and hard “g” pronunciations. The official language of the BIPM SI Brochure is French, but includes an English translation. The common French pronunciation of giga also uses the soft g sound.

Gigawatt.  A great example where popular culture portrays technical information is the classic movie scene from the film Back to the Future (1985) where characters Dr. Emmet Brown (Christopher Lloyd) and Marty McFly (Michael J. Fox) discuss “1.21 gigawatts” using the soft “g”. The SI helps the international scientific community communicate written technical information effectively and overcome the variations of language, including spelling and pronunciation. Verbal pronunciation of SI terminology is purposefully not addressed most SI style guides, which focus on written communication.




Created January 13, 2010, Updated April 21, 2021