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. SI prefixes are an alternative to the scientific “powers-of-ten” notation. Example: 5.2 × 10³ g becomes 5.2 kg. Compound prefix names or symbols are not permitted. Example: nm (nanometer), NOT mμm (millimicrometer). 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.

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).

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.

Resources

• SI Prefixes (BIPM)
• Commonly Used Metric System Units, Symbols, and Prefixes (US Metric Association) 
• Are you pronouncing ‘kilogram’ correctly (Australian Broadcasting Corporation)
   

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