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SI Units
As of August 16, 2023, the physics.nist.gov historic SI Units site has permanently retired. This page and complete Metric (SI) Program contains current SI information. Contact TheSI [at] nist.gov (TheSI[at]nist[dot]gov) with comments, questions or concerns.
The SI rests on a foundation of seven (7) defining constants: the cesium hyperfine splitting frequency, the speed of light in vacuum, the Planck constant, the elementary charge (i.e. the charge on a proton), the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specified monochromatic source. NIST provides values and a searchable bibliography for the fundamental physical constants. Definitions of all seven (7) SI base units are expressed using an explicit-constant formulation and experimentally realized using a specific mises en pratique (practical technique).
The seven SI base units, which are comprised of:
- Length - meter (m)
- Time - second (s)
- Amount of substance - mole (mol)
- Electric current - ampere (A)
- Temperature - kelvin (K)
- Luminous intensity - candela (cd)
- Mass - kilogram (kg)
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, which are illustrated in NIST SP 1247, SI Base Units Relationship Poster. The SI plays an essential role in international commerce and is commonly used in scientific and technological research and development. Learn more about the SI in NIST SP 330 and SP 811.
Useful Definitions
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Washington Monument, Washington, District of Columbia Credit: Adobe StockA quantity in the general sense is a property ascribed to phenomena, bodies, or substances that can be quantified for, or assigned to, a particular phenomenon, body, or substance. Examples are mass and electric charge.
- A quantity in the particular sense is a quantifiable or assignable property ascribed to a particular phenomenon, body, or substance. Examples are the mass of the moon and the electric charge of the proton.
- A physical quantity is a quantity that can be used in the mathematical equations of science and technology.
- A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value.
- The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value. Thus, the numerical value of a particular physical quantity depends on the unit in which it is expressed.
- Example: the value of the height hW of the Washington Monument is hW = 169 m. Here hW is the physical quantity, its value expressed in the unit "meter," unit symbol m, is 169 m, and its numerical value when expressed in meters is 169.
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