First new SI prefixes for over 30 years

The International Bureau of Weights and Measures (BIPM) has formally approved the use of four new SI prefixes to meet the growing needs of science, computing and the increasing amount of online data and to prevent the adoption of unofficial prefix names. The approval of the new prefixes was one of the resolutions of the 27th General Conference on Weights and Measures, which took place on 15-18 November 2022.

The last time that new SI prefixes were added to the metric system was in 1991 when the BIPM added the following prefixes:


The reasons given by the BIPM for expanding the range of new SI prefixes are:

  • to provide new SI prefixes for scientific communities that depend on measurements that are not covered by the current range,
  • to meet the needs of data science in the near future to express quantities of digital information using orders of magnitude in excess of 1024,
  • and to prevent unofficial prefix names being de facto adopted in other communities.

Richard Brown, the head of metrology at the UK’s National Physical Laboratory, came up with the idea for SI prefixes to respond to the unofficial use of prefixes for data storage such as brontobytes and hellabytes. These prefixes did not fit into the SI naming scheme and they started with letters that were already in use for existing prefixes or for other units.

The new prefixes that were approved are:


These prefix names were chosen because they start with letters that have not been used for existing prefixes or units. They use the convention that prefix names for very large numbers end with A and use capital letters for their symbols and prefix names for very small numbers end with O and use small letters for their symbols.

The ronna and ronto prefixes are derived from a combination of “r” plus the Greek ennea for nine plus the common SI suffix of “to” for large numbers or “ta” for small numbers. The number nine relates to the fact that ronna and ronto are 10009 and 1000-9 respectively.

The quetta and quecto prefixes are loosely derived from a combination of “q” plus the Latin decem for ten plus the common SI suffix of “to” for large numbers or “ta” for small numbers. Richard Brown had proposed quecca as the prefix for 1030 but this was changed to quetta. The number ten relates to the fact that quetta and quecto are 100010 and 1000-10 respectively.

The addition of new prefixes improves the metric system’s ability to measure extremes at both ends of the scale and meet the world’s needs for higher numbers. The latest additions should future proof the metric system for the next 20-30 years.

Further reading:

Information about new SI prefixes:

10 thoughts on “First new SI prefixes for over 30 years”

  1. It’s interesting that the New Scientist cites the non-SI units, used for information technology, as having the greatest need for the larger metric prefixes, “the amount of information is projected to hit 175 zettabytes by 2025”.

    As information technology plays an ever-greater role in our lives, isn’t it time for information units, such as the “byte”, and “bit per second” to be officially included in the SI.


  2. Well now – I’d be interested to see what special unit the USA has to concoct for it’s own special way of measuring things now that these prefixes have been added.


  3. m,

    I can see the bit (b) and bit per second (b/s) entering SI but not the byte. The byte is not consistent with the structure of SI, seeing that it relates to the bit by a factor of 8. Also, the BIPM does not create multiple units for the same measurement and byte is a duplication of the bit. Thus the bit and the prefixes would be the way to go.

    Another thing to consider is if with the new prefixes are there now new prefixes added to the binary set? Is there a ronbi (Ri)and a quetbi (Qi) now added to the binary series?


  4. While new prefixes are being added to the SI one of the fundamental units in the SI, the second, will also be redefined using visible light at some point. See this video for more:

    Note at the beginning of the video the slight dig at the USA for being an outlier with its predominant public use of Imperial (well, the American version of Imperial). πŸ˜‰

    Liked by 1 person

  5. @Miles Footpounder 13:57
    In American 10^30 = nonillion, and in English it is quintillion.
    My first thought was “what is a light year in SI notation and why is it not used?”
    9.46 Pm seems to be the answer to the first part, or as the xperts put it 9.66 Tkm. That could surly be rounded out to 10 Pm or rationalised with the parsec? Nothing very sensible nor memorable otherwise. The Tera and Peta (flagged up as misspelt) sits comfortably with computer bits, bytes and flops, and probably the highest any of us will encounter on a daily basis.


  6. @Brian I hope the “xperts” don’t actually use 9.66 Tkm, as you undoubtedly know, the SI rules don’t allow for multiple prefixes.

    That’s why the “kilogram” needs to be renamed: Because the “base unit of mass” in the SI can’t use the same prefix rules as the other units.

    With a renamed “kilogram” (I suggest “klug” but I don’t care if it is called something else) then the SI ideal of only one unit name for mass could be realized:

    1000 grams = 1 klug = 1 kg (base unit of mass)
    1 gram = 1 milliklug = 1 mkg
    1 milligram = 1 microklug = 1 ukg
    1 microgram = 1 nanoklug = 1 nkg

    Notice that with a properly named base mass unit, there is no need for “gram” although I’m not suggesting deprecating it.

    I have visited multiple web sites and even pro-metric web sites where they mistakenly claim that the “gram” is the base unit of mass in the SI system.

    Admittedly, it would (as usual) take years before it caught on, but at least it would have a chance to be a properly named unit along with all the others.

    Other SI units have already been renamed in the past:
    Cycles per second -> Hertz
    Centigrade -> Celsius


  7. @David Summers
    I am not sure where I saw that Terra km. Going back on my research history it is most definitely petametres today, so maybe that one has been corrected.


  8. I keep finding YouTube videos and articles on the internet that keep using “micron” and “bar” instead of “micrometre” and some multiple of the pascal. And then there are the occurrences of “degrees Kelvin” instead of just “kelvins”. Or “half a kilometre” instead of “500 metres”.

    So, the world still has a way to go to get SI 100% right even outside the main muddle spots of Canada, the USA, and the UK. But every little bit helps where we can get it, I suppose, yes? πŸ˜‰


  9. Ezra, car tyre pressures in the UK are measured in either bars or psi, not kPa and it is really annoying to see the tyre air machines resetting to psi after a few minutes.


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