SI Units

SI Units

The International System of Units is the modern form of the metric system and is the world's most widely used system of measurements, used in both everyday commerce and science. It comprises a coherent system of units of measurement built around seven base unit , 22 named and an indeterminate number of unnamed coherent derived unit, and a set of prefixes that act as decimal-based multipliers. It is part of the International System of Quantities

The standards, published in 1960 as the result of an initiative started in 1948, are based on the meter -kilogram-second (MKS) system, rather than the centimeter-gram-second (CGS) system, which, in turn, had several variants. The SI has been declared to be an evolving system; thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The 25th CGPM meeting in the fall of 2014 will change the definitions of some base units, particularly the kilogram.

The driving force behind the development of the Système international was the diversity of units that had sprung up within the CGS system of units and the lack of coordination between the various disciplines that made extensive use of units of measurement. 

The system has been adopted by most countries in the developed world, though within English-speaking countries, the adoption has not been universal. In the United States metric units are not commonly used outside of science, medicine and the government; however, United States customary units are officially defined in terms of SI units. The United Kingdom has officially adopted a partial metrication policy, with no intention of replacing imperial units entirely. Canada has adopted it for most governmental and scientific purposes, but imperial units are still legally permitted and remain in common use throughout many sectors of Canadian society, particularly in the building trade and railway sectors.