Person holding wallet card showing the values of fundamental constants being used for revising the International System of Units, the modern metric system.
Credit: Stoughton/NIST
The units in the revised SI are 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. Learn more about each of these “invariants of nature” and how they come into play in the revised SI.

Meet The Constants

Scientists Voted on Metric Makeover

After decades of groundbreaking laboratory work, the world’s scientific and technical community has redefined four of the seven base units for the International System of Units (SI). A vote to adopt the change happened on November 16, 2018, at Versailles, France, and the change went into effect on May 20, 2019.