The International System of Units

The International System of Units (abbreviated SI from French: Le Système international d'unités) is the modern form of the metric system. 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. The standards, published in 1960, are based on the metre-kilogram-second system, rather than the centimetre-gram-second 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 as the precision of measurements improves. SI is the world's most widely used system of measurement, used in both everyday commerce and science.

The system has been nearly globally adopted. Burma, Liberia and the United States have not adopted SI units as their official system of weights and measures. The U.S. does not commonly use metric units outside of science, medicine, and the government, but has officially defined its customary units 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 purposes but imperial units are still legally permitted and remain in common use throughout a few sectors of Canadian society, particularly in the buildings, trades and railways sectors.