Expansion to the SI prefix range

In November 2022, measurement scientists and government representatives from around the world voted at the General Conference on Weights and Measures (CGPM) to expand the range of prefixes used within the International System of Units (SI).

This decision means that four new prefixes (two at the top end of the SI range and two at the bottom end of the SI range) can now be used to express measurements worldwide. This is the first expansion to the SI prefix range since 1991.  

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What are the new SI prefixes and why do we need them? 

The proposal, led by Prof Richard Brown, Head of Metrology at NPL, recommended the four new names. 

The new names for very large numbers are: 

• ronna (symbol R) for 10²⁷ or 1 000 000 000 000 000 000 000 000 000  

• quetta (symbol Q) for 10³⁰ or 1 000 000 000 000 000 000 000 000 000 000 

They are joined by their tiny counterparts: 

• ronto (symbol r) for 10⁻²⁷ or 0.000 000 000 000 000 000 000 000 001 

• quecto (symbol q) for 10⁻³⁰ or 0.000 000 000 000 000 000 000 000 000 001 

The change was largely driven by the growing requirements of data science and digital storage, which is already using prefixes at the top of the existing range (yottabytes and zettabytes, for expressing huge quantities of digital information). The prefixes for very small numbers are useful for quantum science and particle physics. However, these can be used with any SI unit, for example in the future we can be expected to talk about ronnametres and quettagrams.

The SI prefixes are an essential part of the International System of Units, allowing us to extend the use of the SI as widely as possible, and ensuring the effective and unified communication of measurements across technical disciplines, without the requirement to use or create non-SI units.

When our measurements get very large (or very small) the numbers we use to express these measurements using SI base units or SI derived units also become very large (or very small) and so more difficult to interpret. For instance 0.0045 m or 34 000 000 000 W. 

The SI prefixes can be attached to the SI base units or SI derived units to form new multiples (or sub-multiples) of these units. When expressing the same measurement, changing the size of the unit has the effect of also changing the number used with the unit. If a suitable SI prefix is used this makes the measurement much easier to interpret and communicate. For instance 0.0045 m becomes 4.5 mm (millimetres), because 1 mm = 0.001 m, and 34 000 000 000 W becomes 34 GW (gigawatts), because 1 GW = 10⁹ W.