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The International System of Units (SI), the modern metric system of measurement, has long been the language universally used in science. The SI was established in 1960 by the 11th General Conference on Weights and Measures (CGPM). The CGPM is the international authority that ensures wide dissemination of the SI and modifies the SI as necessary to reflect the latest advances in science and technology. To learn more about SI, CGPM and their backgrounds, visit: National Institute of Standards and Technology.

 The Seven SI Base Units Physical Quantity Dimension Unit Name Unit Symbol Mass M kilogram kg Length L meter m Time T second s Temperature Θ kelvin K Amount of Substance N mole mol Electric Current I ampere A Luminous Intensity J candela cd The Two SI Supplementary Units Physical Quantity Dimension Symbol Unit Name Unit Symbol Plane Angle α radian rad Solid Angle Ω steradian sr
* These are non-dimensional units that are sometimes included in the dimensional expression of a unit for clarity purposes.
 meter: The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 second. [17th CGPM (1983), Resolution 1] kilogram: The kilogram is the unit of mass equal to the mass of the international prototype of kilogram. [1st CGPM (1889), 3rd CGPM (1901)] second: The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levers (F=4, mF=0 to F=3, mF=0) of the ground state of the cesium 133 atom. [13th CGPM (1967)] ampere: The ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2x10-7 newton per meter of length. [9th CGPM (1948), Resolution 2 and 7] kelvin: The kelvin, unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. [13th CGPM (1967), Resolution 4] mole: mount of substance of a system which contains as many elementary entities as there are atoms in .012 kg of carbon 12 (about 6.022x1023 atoms). When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. [14th CGPM (1971), Resolution 3] In this definition, it is understood that the carbon 12 atoms are unbound, at rest and in their ground state. candela: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian. [16th CGPM (1979), Resolution 3]

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CNC Machining Design Guide

Optimize your designs, reduce machining time, and lower your costs.

STEM Career Outlook

Wages, employment opportunities, and growth projections for STEM jobs.

3D Scanners

A white paper to assist in the evaluation of 3D scanning hardware solutions.

Selecting the Right 3D Printer

Discover how to choose the right 3D printer for your needs and the key performance attributes to consider.