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SI Prefix Progress

"The SI is not a static convention, but evolves following developments in the science of measurement" BIPM SI Brochure (9th Edition)
Credit: BIPM SI Brochure (9th Edition)

In December 2022, the BIPM SI Brochure (9th Edition, December 2022, V2.01) was published to include the four new International System of Units (SI) prefixes recently adopted by the General Conference on Weights and Measures (CGPM) in November 2022. The SI prefixes were expanded from 20 to 24 with the addition of ronna, ronto, quetta, and quecto. Technological advancements continue to drive SI updates.  The need for a coherent measurement system for all science, technology, engineering, and mathematics (STEM) fields is more important than ever. It was determined that the previous range of SI prefixes was not meeting the needs of the scientific community, especially data science with the need to express digital information quantities in excess of yotta (1024).

New SI Prefixes

Function

Prefix Name

Symbol

Multiplying Factor

Multiples

quetta

Q

1030

ronna

R

1027

Submultiples

ronto

r

10-27

quecto

q

10-20

Taking action now helps prevent “unofficial” prefix names from becoming incorporated into technical writing practice. Prior to this prefix expansion, the last major SI redefinition went into effect on May 20, 2019. Since the first eight prefixes were adopted by the CGPM in 1889, there have been five subsequent prefix expansions. This chronological summary highlights these developments.

 Year

Description

Prefix Total

1795

Eight original SI prefixes were officially adopted: deca, hecto, kilo, myria, deci, centi, milli, and myrio, derived from Greek and Latin numbers. Initially, all prefixes were represented by lowercase symbols.

8

1889

The first General Conference on Weights and Measures (CGPM) approves the 8 prefixes for use.

8

1960

Two prefixes were made obsolete:  myria and myrio. Six prefixes were added. Three for forming multiples:  mega, giga, and tera.  Three for forming submultiples: micro, nano, and pico.

12

1964

Two prefixes for forming submultiples were added:  femto and atto.  This created an imbalanced situation, where there were more prefixes for small quantities.

14

1975

Two prefixes for forming multiples were added:  peta and exa.

16

1991

Four prefixes were added. Two for forming multiples:  zetta and yotta.  Two for forming submultiples: zepto and yocto.

20

2022

Four prefixes were added.  Two for forming multiples:  ronna and quetta.  Two forming submultiples:  ronto and quecto.

24

Currently, 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 except for these three exceptions: kilo (k), hecto (h) and deka (da).

SI style and usage focuses on written communication. Values of quantities are expressed using Arabic symbols for numbers paired with a unit symbol, often with a prefix symbol that modifies unit magnitude. When added to a unit name, SI prefixes create larger or smaller units by factors that are powers of 10. For example, combine the prefix R (ronna) with the unit g (gram) to form 1 Rg (ronnagram) to indicate 1 000 000 000 000 000 000 000 000 000 g.

NIST OWM encourages downloading the BIPM SI Brochure (9th Edition, December 2022, V2.01) and updating laboratory Quality Management System references, training materials, textbook and webpage content, and related citations. NIST Special Publication (SP) 330, The International System of Units (SI) and NIST SP 811, The NIST Guide for the use of the International System of Units (SI) are two primary SI writing guides. These and other NIST reference publications will be updated to reflect SI updates as new editions are released.  For additional SI (Metric System) writing and prefix guidance, please contact TheSI [at] nist.gov

Released January 4, 2023, Updated January 9, 2023