- m - Meter - Length
- s - Second - Time
- mol - Mole - Amount of Substance
- A - Ampere - Electric Current
- K - Kelvin - Temperature
- cd - Candela - Luminous Intensity
- kg - Kilogram - Mass
The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. The International Treaty of the Meter was signed in Paris on May 20, 1875 by seventeen countries, including the United States and is now celebrated around the globe as World Metrology Day. NIST provides official U.S. representation in the various international bodies established by the Meter Convention: CGPM - General Conference on Weights and Measures; CIPM - International Committee for Weights and Measures; and BIPM - The International Bureau of Weights and Measures.
The SI is made up of 7 base units that define the 22 derived units with special names and symbols. The international prototype of the kilogram is the only remaining artifact used to define a base unit of the SI (rather than definition by a fundamental constant). The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. The prototype kilogram is a cylinder with a height and diameter of about 39 mm and is made of an alloy of platinum and iridium. The SI plays an essential role in international commerce and is the commonly used in scientific and technological research and development. Learn more about the SI in NIST SP 330 and SP 811.