International System of Units
The International System of Units, known by the abbreviation SI, anchors its entire structure on seven specific base units. These units define fundamental physical quantities like time, length, and mass. The second measures time through the hyperfine transition frequency of caesium-133 atoms. A single second equals exactly 9,192,631,770 periods of radiation from this atomic transition. Length relies on the metre, defined as the distance light travels in vacuum during a fraction of that same second. Mass uses the kilogram, which now derives its value from the Planck constant rather than a physical object. Electric current flows as the ampere, representing the flow of elementary charges per second. Temperature measures thermodynamic heat using the kelvin, fixed by the Boltzmann constant. Amount of substance counts entities via the mole, tied to the Avogadro constant. Luminous intensity defines brightness with the candela based on monochromatic radiation at a specific frequency.
Scientists construct twenty-two coherent derived units by combining powers of the seven base units. Velocity becomes metres per second while force transforms into newtons expressed as kilograms times metres divided by seconds squared. Pressure takes the form of pascals, calculated as one newton applied over one square metre. Energy measures work or heat in joules, equivalent to watts multiplied by seconds. Power itself is measured in watts, representing energy transfer over time. The system allows unlimited additional units formed through products or quotients of these base elements. Decimal prefixes modify these units to create multiples and sub-multiples across wide ranges. A prefix like kilo- represents a factor of 1000, turning metres into kilometres for driving distances. Another prefix such as milli- denotes a thousandth, creating millimetres from standard metres. These prefixes never combine; a millionth of a metre remains a micrometre rather than a millimillimetre. The International Bureau of Weights and Measures specifies exactly 24 metric prefixes ranging from 10 to the power of negative 30 up to positive 30.
The concept of a unified measurement system emerged roughly a century before the SI was officially born. James Clerk Maxwell and William Thomson developed the centimetre, gram, second system during the 1860s under the British Association for the Advancement of Science. This CGS system formalized how related units could define derived quantities without extra factors. In 1875, seventeen nations signed the Metre Convention to establish international cooperation on metrology standards. That treaty created the General Conference on Weights and Measures to oversee definitions and rules. Giovanni Giorgi resolved electrical unit inconsistencies in 1901 by proposing a fourth base unit alongside length, mass, and time. Electric current became that new ampere, allowing voltage and resistance to derive logically from physics laws. The MKSA system combining these four units received approval in 1946. A 1948 study commissioned by the 9th CGPM assessed global measurement needs for science and education. The 10th CGPM defined an international system using six base units including the degree Kelvin and candela. Finally, the 11th CGPM adopted the International System of Units in 1960 as the modern form of the metric system.
Physical artifacts once served as the foundation for defining base units like the kilogram. The International Prototype of the Kilogram was a platinum-iridium cylinder stored near Paris. Divergence occurred between this prototype and its official copies during periodic verifications in the early 2000s. Copies had noticeably increased in mass relative to the original artifact over decades. This instability undermined reliability across scales from atomic particles to astrophysical measurements. Scientists proposed fixing numerical values for seven constants of nature instead of relying on physical objects. These constants include the speed of light, Planck constant, elementary charge, Boltzmann constant, and Avogadro constant. The new definitions were adopted at the 26th General Conference on Weights and Measures on the 16th of November 2018. They came into effect globally on the 20th of May 2019. The European Union implemented these changes through Directive EU 2019/1258 shortly thereafter. All issues regarding loss or damage to physical prototypes disappeared with this shift. Base units now derive exclusively from fixed values assigned to natural constants rather than manufactured standards.
The International System of Units holds official status in nearly every country worldwide. Science, technology, industry, and commerce rely on this single decimal system for daily operations. Three international organizations manage SI standards under the terms of the Metre Convention established in 1875. The General Conference on Weights and Measures makes all decisions concerning unit definitions. The International Committee for Weights and Measures oversees implementation and technical work. The International Bureau of Weights and Measures publishes the official SI Brochure in French and English. This document collects all recommendations and updates periodically to reflect scientific progress. Some nations like the United States, Canada, and the United Kingdom continue using customary systems alongside SI. Despite partial usage of imperial measures, SI remains the preferred language for global trade and research. The only other widespread measurement systems are imperial and US customary variants. These alternative systems define their units in terms of SI measurements today.
Certain non-SI units persist because they remain deeply embedded in history and culture. The hour, minute, and day measure time but use conversion factors that are not powers of ten. Angles utilize degrees, arcminutes, and arcseconds despite SI defining radians as the standard derived unit. Volume often uses the litre even though it is not an SI base unit. Mass frequently employs the tonne rather than kilograms for large quantities. Energy calculations sometimes rely on the electronvolt instead of joules. The CIPM compiled a list of these accepted non-SI units for use alongside SI standards. Conversion between some of these units requires factors outside simple decimal multiples. For instance, one hour equals 3600 seconds while one degree represents approximately 17.5 milliradians. Other metric units exist completely outside any system recognized by the SI. Examples include the sverdrup for ocean currents and the darcy for permeability. Mixing information with unit names like watt-peak or atomic second violates unacceptable practices within the official framework.
Common questions
What are the seven base units of the International System of Units?
The International System of Units anchors its entire structure on seven specific base units. These units define fundamental physical quantities like time, length, and mass through the second, metre, kilogram, ampere, kelvin, mole, and candela.
When did the 11th CGPM adopt the International System of Units in 1960?
The 11th General Conference on Weights and Measures adopted the International System of Units in 1960 as the modern form of the metric system. This adoption followed a 1948 study commissioned by the 9th CGPM that assessed global measurement needs for science and education.
On what date did the new definitions based on natural constants come into effect globally?
Scientists proposed fixing numerical values for seven constants of nature instead of relying on physical objects to ensure stability across scales from atomic particles to astrophysical measurements. The new definitions were adopted at the 26th General Conference on Weights and Measures on the 16th of November 2018 and came into effect globally on the 20th of May 2019.
Which three international organizations manage SI standards under the terms of the Metre Convention established in 1875?
Three international organizations manage SI standards under the terms of the Metre Convention established in 1875. These bodies are the General Conference on Weights and Measures, the International Committee for Weights and Measures, and the International Bureau of Weights and Measures.
What is the definition of the second in the International System of Units?
The second measures time through the hyperfine transition frequency of caesium-133 atoms. A single second equals exactly 9,192,631,770 periods of radiation from this atomic transition.