Brynn Hibbert receives funding from ARC, International Union of Pure and Applied Chemistry (IUPAC). He is Past President of the Royal Society of New South Wales, Past Chair of the Analytical Division of IUPAC and present Secretary of the IUPAC Interdivisional Committee on Terminology Nomenclature and Symbols (ICTNS). We measure stuff all the time – how long, how heavy, how hot, and so on – because we need to for things such as trade, health and knowledge. But making sure our measurements compare apples with apples has been a challenge: how to know if my kilogram weight or metre length is the same as yours. Attempts have been made to define the units of measurement over the years. But today – International Metrology Day – sees the complete revision of those standards come into play. You won’t notice anything – you will not be heavier or lighter than yesterday – because the transition has been made to be seamless. Just the definitions of the seven base units of the SI (Système International d’Unités, or the International System of Units) are now completely different from yesterday. Humans have always been able to count, but as we evolved we quickly moved to measuring lengths...