Metrication in the United States

Metrication is the process of introducing what is now the International System of Units, also known as SI units, to replace a jurisdiction's traditional measuring units. U.S. customary units have been defined in terms of metric units since the 19th century, and, according to United States law, the SI has been the "preferred system of weights and measures for United States trade and commerce" since 1975.

Source: Wikipedia — Metrication in the United States (CC BY-SA 4.0)