21/05/2026
This is a big day in our world! It’s not only World Metrology Day, but also the 151st anniversary of an important historical event called the Meter Convention. On this day in 1875, the U.S. and 16 other countries agreed on international definitions for the unit of length (the meter) and the unit of mass (the kilogram).
The meter and kilogram were the first common measurement units for international trade and science. International definitions for the second, mole, kelvin, and other units followed, leading to the International System of Units (SI), or the modern metric system.
Why are we so excited about this? It’s because NIST is our nation’s national measurement science institute. Ever wonder how you get accurate time on the computer you may be using right now? Or why you can trust the readings on the tape measures you buy at the hardware store? Or how GPS can provide door-to-door directions? NIST’s science and technology help make all these things happen.
It’s a party today for measurement science institutes around the world. The Meter Convention is also the birthday of the International Bureau of Weights and Measures (BIPM) in France, where a prototype kilogram and meter bar were stored. At BIPM, NIST and other national measurement institutes work together to advance measurement science, helping to support commerce and science around the world.
Learn more: https://www.nist.gov/si-redefinition/kilogram-focus-history