Quantitative measurements are indispensable in the conduct of science. This is as true for medicine as it is for chemistry or physics. The close interrelationships between the many branches of science and the international character of science and medicine require that a single, worldwide system of measurements be employed. The international system of units (SI) as recognized by the Conference Générale des Poide et Mesures has now been adopted by most countries and is being used in most scientific areas, including engineering.
This system has seven base units and two supplementary units of measurements (Table 1).
From the seven base units, all other measurements can be derived. As examples, volume, area, speed, pressure, force, and work are obtained by algebraically combining with multiplication and division two or more base units. When the base units are combined algebraically, the derived unit is said to be rational, and no prefix is needed
Barclay WR. Standardizing Units of Measurements. JAMA. 1976;236(17):1981. doi:10.1001/jama.1976.03270180057027