International System of Units

The International System of Units (abbreviated SI from ) is the modern form of the metric system and is the world’s most widely used system of measurement, used in both everyday commerce and science. It comprises a coherent system of units of measurement built around seven base units, 22 named and an indeterminate number of unnamed coherent derived units, 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 metre–kilogram–second (MKS) system, rather than the centimetre–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 final quarter of 2014 planned to consider a proposal to change the definitions of some base units, particularly the kilogram. The motivation for 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. In addition to defining a new realisation of the metric system, the General Conference on Weights and Measures, an organisation set up by the Convention of the Metre in 1875, succeeded in bringing together many international organizations to agree not only the definitions of the SI, but also rules on writing and presenting measurements in a standardised manner around the globe. 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, medical and scientific purposes, but imperial units, legally defined in terms of SI units, are still legally permitted and remain in common use throughout many sectors of Canadian society, particularly in the building trade and railway sectors. Retail grocery for the most part is weighed in SI units, and weather reading and reporting, driving, and the sale of gasoline (petrol) are exclusively in SI.