International System of Units (SI)

The principle behind the International System of Units is to provide the same values for measurements such as length, weight, and time no matter where in the world measurement is performed. The units used in this system are referred to as “SI units.” The system was established at the 1960 General Conferences on Weights and Measures (CGPM). The abbreviation “SI” stands for “Le Système International d'Unités.”

The International System of Units comprises the following three categories.

Base Units

Amount	Unit name	Unit symbol	Definition
Length	Unit name Meter	Unit symbol m	Definition The distance traveled by light in a vacuum in 1/299792458 second.
Weight	Unit name Kilogram	Unit symbol kg	Definition This is the unit for weight. The mass of the international prototype kilogram.
Time	Unit name Second	Unit symbol s	Definition The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Current	Unit name Ampere	Unit symbol A	Definition The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10-7 newtons per meter of length.
Thermodynamic temperature	Unit name Kelvin	Unit symbol K	Definition 1/273.16 of the thermodynamic temperature of the triple point of water.
Substance amount	Unit name Mole	Unit symbol mol	Definition The amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. (Limited to objects with clarified composition.) Elementary entities are subatomic particles that compose matter and energy.
Luminosity	Unit name Candela	Unit symbol cd	Definition The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Supplementary Units

Amount	Unit name	Unit symbol	Definition
Plane angle	Unit name Radian	Unit symbol rad	Definition Radian describes the plane angle subtended by an arc of a circle with the same length as the radius of that circle corresponds to an angle of 1 radian.
Solid angle	Unit name Steradian	Unit symbol sr	Definition A steradian is a solid angle at the center of a sphere subtending a section on the surface equal in area to the square of the radius of the sphere.

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Derived Units

Derived units are a combination of base units and supplementary units and the mathematical symbols of multiplication and division.

Amount	Unit name	Unit symbol
Area	Unit name Square meter	Unit symbol m²
Volume	Unit name Cubic meter	Unit symbol m³
Speed	Unit name Meter per second	Unit symbol m/s
Acceleration	Unit name Meter per second squared	Unit symbol m/s²
Wavenumber	Unit name Reciprocal meter	Unit symbol m^-1
Density	Unit name Kilogram per cubic meter	Unit symbol kg/m³
Current density	Unit name Ampere per square meter	Unit symbol A/m²
Magnetic field strength	Unit name Ampere per meter	Unit symbol A/m
Concentration (of amount of substance)	Unit name Mole per cubic meter	Unit symbol mol/m³
Specific volume	Unit name Cubic meter per kilogram	Unit symbol m³/kg
Luminance	Unit name Candela per square meter	Unit symbol cd/m²

Some derived units are given unique names.

Amount	Unit name	Unit symbol	Composition
Frequency	Unit name Hertz	Unit symbol Hz	Composition 1Hz=1s^-1
Force	Unit name Newton	Unit symbol N	Composition 1N=1kg･m/s²
Pressure, stress	Unit name Pascal	Unit symbol Pa	Composition 1Pa=1N/m²
Energy, work, amount of heat	Unit name Joule	Unit symbol J	Composition 1J=1N･m
Power, radiant flux	Unit name Watt	Unit symbol W	Composition W=1J/s
Electric charge, amount of electricity	Unit name Coulomb	Unit symbol C	Composition 1C=1A･s
Electric potential/electric potential difference, voltage, electromotive force	Unit name Volt	Unit symbol V	Composition 1V=1J/C
Resistance (electrical)	Unit name Ohm	Unit symbol Ω	Composition 1Ω=1V/A
Conductance (electrical)	Unit name Siemens	Unit symbol S	Composition 1S=1Ω^-1
Magnetic	Unit name Weber	Unit symbol Wb	Composition 1Wb=1V･s
Magnetic flux density, magnetic induction	Unit name Tesla	Unit symbol T	Composition 1T=1Wb/m²
Inductance	Unit name Henry	Unit symbol H	Composition 1H=1Wb/A
Celsius temperature	Unit name Degree Celsius	Unit symbol °C	Composition 1t=T-To
Luminous flux	Unit name Lumen	Unit symbol lm	Composition 1lm=1cd･sr
Illuminance	Unit name Lux	Unit symbol lx	Composition 1lx=1lm/m²

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Reference Information

SI unit prefixes indicating integer powers of ten

Factor	Prefix	Symbol	Factor	Prefix	Symbol
10¹⁸	Prefix exa	Symbol E	10^-1	Prefix deci	Symbol d
10¹⁵	Prefix peta	Symbol P	10^-2	Prefix centi	Symbol c
10¹²	Prefix tera	Symbol T	10^-3	Prefix milli	Symbol m
10⁹	Prefix giga	Symbol G	10^-6	Prefix micro	Symbol µ
10⁶	Prefix mega	Symbol M	10^-9	Prefix nano	Symbol n
10³	Prefix kilo	Symbol k	10^-12	Prefix pico	Symbol p
10²	Prefix hecto	Symbol h	10^-15	Prefix femto	Symbol f
10	Prefix deka	Symbol da	10^-18	Prefix atto	Symbol a

Non-SI units

Amount	Unit name	Unit symbol	Definition
Time	Unit name Minute	Unit symbol min	Definition 1min=60s
	Unit name Hour	Unit symbol h	Definition 1h=60min
	Unit name Day	Unit symbol d	Definition 1d=24h
Plane angle	Unit name Degree	Unit symbol °	Definition 1°= (π/180) rad
	Unit name Minute	Unit symbol ′	Definition 1′= (1/60) °
	Unit name Second	Unit symbol ″	Definition 1″= (1/60) ′
Volume	Unit name Liter	Unit symbol l, L	Definition 1l=1dm³
Weight	Unit name Metric ton	Unit symbol t	Definition 1t=10³kg

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Evolution of SI Measurement Systems

The formation of the International System of Units, known as SI from the French Systeme International s’Unites, is the result of an international calibration towards a standardized system of measurements. The First Industrial Revolution brought about the rise of the textile industry and significant developments in metallurgy and coal mining.

However, most regions of the world relied on local units and standards that varied significantly from one place to another. Thus, the French introduced the SI measurement system as a revolutionary idea aimed at creating a uniform and logical system of measurement to appease the growing complexities of science, engineering, and trade, all of which demanded standardization.

The SI system went through several iterations between the 18th century and the 1960s before it was formally established during the 11th General Conference on Weights and Measures. This particular event defined the SI based on the physical constants of nature (seen in the table above) instead of tangible objects, thus ensuring greater precision and universality when using distance sensors.

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Real-World Applications of SI Units

Since it provides universal and standardized measurements, the international unit system is widely used in modern industries and research and development settings. It enables effective communication between virtually everyone involved in the manufacturing of scientific processes.

Here are some key areas in which the SI system is used:

Engineering and Construction

Precision is the foundation of safety, efficiency, and innovation in engineering and construction, and the introduction of the international unit system enabled engineers worldwide to share and apply knowledge seamlessly. For example, architects rely on meters and kilograms to calculate structural integrity, while electrical engineers and technicians rely on amperes to design adequate and safe electrical systems.

Science and Research

The international unit system is the universal language of science since it enables researchers and scientists (and everyone else involved in manufacturing and R&D) to share and apply knowledge amongst themselves, regardless of the language they speak or the region they come from.

With its based and derivative units, the SI system became an indispensable tool for a whole range of scientific measurements, from measuring chemical reactions in moles to observing cosmic phenomena in kilometers and seconds with measurement sensors.

Healthcare and Medicine

Precision and accuracy are also very important in the medical and healthcare industries and often mean the difference between life and death. Dosages and other fluid measurements are mostly expressed in milliliters, or a thousandth of a liter, which is derived from a unit of volume (cubic meter).

One milliliter equals one cubic centimeter, and it’s a critical measurement in fluid administration, such as administering crystalloid solutions (IV fluids) and intravenous or intramuscular injections. Kilograms, the international unit of weight, is also important in the calculation of medication dosages, usually milligrams or milliliters of medication per kilogram of body weight.

Industry and Commerce

International trade and industrial efficiency also rely on the international system of measurement since it enables seamless interactions and transactions across the globe. The SI units are important in ensuring that products meet strict specifications for weight, dimensions, and production times.

For example, the food industry measures ingredients in grams and milliliters, while the electronics sector operates on volts and amperes (amps), and commerce relies on the SI system to standardize the measurement of goods, shipping volumes, and transportation distances—which are important for logistics and supply chain management.

Summary

The international unit system is more than just a technical framework; it’s a universal language of measurement that spans entire disciplines, cultures, and industries. Its evolution from a revolutionary idea to a universal standard reflects the societal endeavor for precision, understanding, and cooperation.

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