spaceSpace and Physics

The Earth Now Weighs Six Ronnagrams, The Sun Two Thousand Quettagrams

New prefixes have been agreed on for the International System of Units, providing easier ways to describe the enormous and the tiny.


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

Sun, Earth and Moon
We used to need powers of ten to express their masses, but now we can express them with the new ronna and quetta prefixes. Image credit: Aphelleon/

It’s a very big universe we live in, and we need big units to describe it. The body that defines our units has come to the rescue, adding the prefixes ronna and quetta for things that are just too big to be comfortably measured in yottas. On the other hand, for when you want to discuss the mind-bendingly small, ronto and quecto are now available.

Even the residents of the USA, Myanmar and Liberia, the last holdouts against the metric system, are familiar with the prefixes kilo and mega for a thousand or million standard units, like grams or meters. In the other direction, their counterparts are milli and micro.


The extremes are used less often, but until last week the largest prefix was yotta, for 1024 units, and the smallest was yocto (10-24). Now, however, the 27th General Conference on Weights and Measures, responsible for oversight of the metric system, have added two additional prefixes at each end, extending the span that can be easily described by a factor of a million in both directions.

Although their roots go back to the French Revolution, International System of Units (SI) units were established in 1960, with an initial range from atto to tera (10-18-1018). As our grasp of the universe grew, extra prefixes were added in 1964 and 1991. 

The prefixes apply to seven base units: the second, meter, (kilo)gram, ampere, kelvin, mole, and candela. Upon these are built derived units, such as the volt or the Newton. However, other units, not under the General Conference’s control, have copied the terminology. The fact the world is now producing so much data that the yottabyte will soon be insufficient to describe it was one of the motivators to expand. 

Dr Richard Brown of the UK’s National Physical Laboratory led the charge to add the units after seeing references to brontobytes and hellabytes – prefixes with no official recognition. It’s important the General Conference provide formal definitions, as otherwise one nation’s bronto could equal another’s hella, leading to exactly the sort of disasters SI units were designed to avoid. If you think that is unlikely, consider the tale of the spaceship lost because of confusion between metric and imperial units.


The General Conference chose not to go with the existing informal prefixes because B and H are already in use in ways that would be confusing if the prefixes were shortened to a single letter like existing ones.

“The only letters that were not used for other units or other symbols were R and Q,” Brown told AFP

This does raise the question of what will be done when these prefixes too become insufficient. Will we have to resort to letters from languages with other alphabets? It’s a tricky question, but on current form we have almost thirty years to solve it.

Meanwhile perhaps the best of examples of the new prefixes’ usefulness comes from mass. The Earth contains 6 ronnagrams (6x1027 grams), while the Sun is almost two thousand quettagrams (2x1033 grams). However, if you wish to discuss the mass of our galaxy you either need to revert to powers of ten (2.3x1015 quettagrams), in which case you may as well just measure it in grams, or use units such as solar masses. 


At the other end of the spectrum, an electron is just shy of a full rontogram, or about 911 quectograms (9x10-28 grams).


spaceSpace and Physics
  • tag
  • measurement,

  • SI units,

  • metric prefixes,

  • ronna,

  • quetta,

  • quecto