SI stands for the International System of Units, also known as the metric system.

This November, in Versailles, France, representatives from 57 countries are expected to make history.  They will vote to dramatically transform the international system that underpins global science and trade. This single action will finally realize scientists’ 150-year dream of a measurement system based entirely on fundamental properties of nature.

The International System of Units, informally known as the metric system—the way in which the world measures everything from coffee to the cosmos—will change in a way that is more profound than anything since its establishment following the French Revolution.  

It will be a turning point for humanity.



The SI — the modern metric system — has seven base units from which all other measurement units can be derived. In November 2018, four of them — the kilogram, kelvin, ampere and mole — are slated to be redefined in terms of constants of nature. The remaining three — the second, meter, and candela — are already based on universal constants.

Click on the SI symbols below for more information (greyed-out items coming soon).

kilogram symbol    candela symbol   kelvin symbol    meter
second symbol   ampere symbol   mole symbol


The units in the revised SI will be based completely on seven unchanging quantities or “universal constants,” including the speed of light, the amount of electric charge in an electron, and the Planck constant. In 2017, exact values were proposed for each of these constants. Learn more about each of these “invariants of nature” and how they will come into play in the revised SI.

Coming Soon!