metric conventions for SI use
and fundamental constants

Based on a 1875 treaty establishing an effort to develop a common set of measurement units to be used world wide, the metric system evolved in 1960 into the Système International d'Unités.  The American Institute of Physics publishes annually a guide for using SI units correctly.

Like most equations, the formulae used to understand the world generally have constants.   The Task Group on Fundamental Physical Constants of the International Council of Science periodically updates a self-consistent set of the best measurements of these constants.  The American Institute of Physics publishes annually this list.