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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. Values of quantities are expressed using Arabic symbols for numbers 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 Symbol Factor Name

larger quantities
or whole units
yotta Y 1024 Septillion
zetta Z 1021 Sextillion
exa E 1018 Quintillion
peta P 1015 Quadrillion
Example: terahertz
T 1012 Trillion
Example: gigawatt
G 109 Billion
mega M 106 Million
Example: kiloliter
k 103 Thousand
Example: hectare
h 102 Hundred
Example: dekameter
da 101 Ten
      100 One
smaller quantities
or sub units

Example: decimeter
d 10-1 Tenth
Example: centigram
c 10-2 Hundredth
Example: milliliter
m 10-3 Thousandth
Example: microgram
μ 10-6 Millionth
Example: nanometer
n 10-9 Billionth
Example: picogram
p 10-12 Trillionth
Example: femtosecond
f 10-15 Quadrillionth
atto a 10-18 Quintillionth
Example: zeptosecond
z 10-21 Sextillionth
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 pronunciation guidance reflects contemporary American English.

Writing. Guidance is provided 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 1137, Metric Style Guide for the News Media.

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FAQ: How do I pronounce the prefix giga? The Merriam-Webster Collegiate Dictionary provides two common pronunciations for the scientific term gigawatt. The soft “g” pronunciation is listed first, followed by the hard "g" pronunciation. Prefix Etymology resources list both soft and hard “g” pronunciations. The official language of the BIPM SI Brochure is French and includes an English translation but provides no pronunciation guidance.




Created January 13, 2010, Updated June 24, 2021