Metric (SI) Prefixes

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.

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.

   * 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.

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.

Resources

• SI Prefixes (BIPM).
• Commonly Used Metric System Units, Symbols, and Prefixes (US Metric Association).
• SI Prefixes and Their Etymologies (US Metric Association).
• Prefix Table and Approval Dates, How Many? A Dictionary of Units of Measurement (Rowlett).
• Are you pronouncing 'kilometre' correctly? (Australian Broadcasting Corporation).
• Becoming Familiar with SI.
• Everyday Estimation.
• Metrication FAQs.
• SI Publications.
• Writing with SI (Metric) Units.