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Metrication (or metrification) is the process of introducing the International System of Units, also known as SI units or the metric system, to replace a jurisdiction's traditional measuring units. U.S. customary units have been defined in terms of metric units since the 19th century,[1] and the SI has been the "preferred system of weights and measures for United States trade and commerce" since 1975 according to United States law.[2] However, conversion was not mandatory and many industries chose not to convert, and U.S. customary units remain in common use in many industries as well as in governmental use (for example, speed limits are still posted in miles per hour). There is government policy and metric (SI) program to implement and assist with metrication, however there is major social resistance for further metrication.[3][4][5]
Although customary units are used more often than metric units in the U.S., the SI system is used extensively in fields such as science, medicine, electronics, the military, automobile production and repair, and international affairs.[6] Post-1994 federal law also mandates most packaged consumer goods be labeled in both customary and metric units.[7]
The U.S. has fully adopted the SI unit for time, the second.[8] The U.S. has a national policy to adopt the metric system.[3] All U.S. agencies are required to adopt the metric system.[9]
In the United States the basic relation officially recognized is that contained in the law of July 28, 1866, and set forth in the Mendenhall order of April 5, 1893; namely...
Also, our scientific and medical communities use metric units almost exclusively.