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Metric (SI) Prefixes

Metric Units of Measurement

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 metric system of measurement, 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 some of the following metric tables, some such multiple and subdivisions have not been included for the reason that these have little, if any currency in actual usage.

In certain cases, particularly in scientific usage, it becomes convenient to provide for multiples larger than 1000 and for subdivisions smaller than one-thousandth. Accordingly, the following prefixes have been introduced and these are now generally recognized.

Purpose Prefix Name Preferred Pronunciation Prefix Symbol Value

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


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 1137, Metric Style Guide for the News Media.

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).

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.


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


Units of Length
10 millimeters (mm) 1 centimeter (cm)
10 centimeters 1 decimeter (dm) = 100 millimeters
10 decimeters 1 meter (m) = 1000 millimeters
10 meters 1 dekameter (dam)
10 dekameters 1 hectometer (hm) = 100 meters
10 hectometers 1 kilometer (km) = 1000 meters


Units of Area
100 square millimeter (mm2) 1 square centimeter (cm2)
100 square centimeter 1 square decimeter (dm2)
100 square decimeters 1 square meter (m2)
100 square meters 1 square dekameter (dam2) = 1 are
100 square dekameters 1 square hectometer (hm2) = 1 hectare (ha)
100 square hectometers 1 square kilometer (km2)


Units of Liquid Volume
10 milliliters (mL) 1 centiliter (cL)
10 centiliters 1 deciliter (dL) = 100 milliliters
10 deciliters 1 liter1 = 1000 milliliters
10 liters 1 dekaliter (daL)
10 dekaliters 1 hectoliter (hL) = 100 liters
10 hectoliters 1 kiloliter (kL) = 1000 liters


Units of Volume
1000 cubic millimeters (mm3) 1 cubic centimeter (cm3)
1000 cubic centimeters 1 cubic decimeter (dm3)
1 000 000 cubic millimeters
1000 cubic decimeters 1 cubic meter (m3)
1 000 000 cubic centimeters
1 000 000 000 cubic millimeters


Units of Mass
10 milligrams (mg) 1 centigram (cg)
10 centigrams 1 decigram (dg) = 100 milligrams
10 decigrams 1 gram (g) = 1000 milligrams
10 grams 1 dekagram (dag)
10 dekagrams 1 hectogram (hg) = 100 grams
10 hectograms 1 kilogram (kg) = 1000 grams
1000 kilograms 1 megagram (Mg) or 1 metric ton(t)


Created January 13, 2010, Updated September 8, 2020