SI units are the units belonging to the International System of Units, the internationally agreed measurement system used to express physical quantities consistently. The principal SI units are the seven base units: second, metre, kilogram, ampere, kelvin, mole and candela. From these, many derived units are constructed, including newton for force, joule for energy, watt for power, pascal for pressure, coulomb for electric charge and volt for electric potential difference.
SI units provide a common quantitative language that allows measurements from different laboratories, instruments and countries to be compared. The modern SI defines its units through exact numerical values of seven defining constants, rather than relying fundamentally on physical artefacts. Consequently, an SI unit is not merely a convenient label attached to a number; it is part of a formally defined measurement system with established relationships, symbols and realisation procedures.
The seven SI base units correspond to seven base quantities. The second is the SI unit of time, the metre is the unit of length, the kilogram is the unit of mass, the ampere is the unit of electric current, the kelvin is the unit of thermodynamic temperature, the mole is the unit of amount of substance and the candela is the unit of luminous intensity. Their modern definitions are interconnected through exact constants.
For example, the metre is defined using the exact value of the speed of light in vacuum and the second, while the kilogram is defined through the exact Planck constant together with the second and metre. The ampere is connected to the exact elementary charge. The kelvin is connected to the exact Boltzmann constant, and the mole is connected to the exact Avogadro constant. These definitions make the SI internally coherent and independent of particular manufactured artefacts.
Derived SI units are produced by combining base units according to mathematical definitions. The newton is kilogram metre per second squared and represents force. The joule is kilogram metre squared per second squared and represents energy or work. The watt is joule per second and represents power. The pascal is newton per square metre and represents pressure. The coulomb is ampere second and represents electric charge.
The volt can be expressed in terms of kilogram metre squared per second cubed ampere and represents electric potential difference. Many derived units have special names because repeatedly writing their base unit expressions would be cumbersome. Understanding these relationships allows scientists to move between physical concepts and fundamental measurements while maintaining dimensional consistency.
SI units use standard symbols and prefixes. The metre has the symbol m, the second s, the kilogram kg, the ampere A, the kelvin K, the mole mol and the candela cd. Prefixes such as kilo, mega, giga, milli, micro and nano represent powers of ten. Symbols are not generally pluralised, and case can be significant. For example, m denotes metre while M as an SI prefix denotes mega. The distinction between unit symbols and quantity symbols is also important. A quantity such as velocity may be represented by a letter such as v, while its SI unit is metre per second. Scientific writing should preserve these conventions because inconsistent symbols can create ambiguity and numerical errors.
SI units are coherent, meaning that derived units are formed from the base units without arbitrary numerical conversion factors. This is useful for equations because physical relationships can be expressed directly in terms of compatible units. Dimensional analysis can identify many calculation errors by checking whether both sides of an equation have the same dimensions. Unit conversion remains necessary when values are supplied in non SI units.
For example, a speed given in kilometres per hour can be converted to metres per second before use in an SI based calculation. Conversion is not merely a formatting operation because incorrect conversion factors can alter physical conclusions. SI coherence therefore provides a framework for consistent calculation, while careful unit handling remains necessary at every stage.
Several units accepted for use with the SI are not SI units in the strict sense. The litre is accepted for use with the SI, as are units such as minute, hour and day for time. Electronvolt is accepted for use in certain scientific contexts and is especially common in particle and atomic physics. Other units, such as mile, inch, pound and calorie, may remain important in specific applications but require conversion when incorporated into SI based calculations.
The existence of accepted non SI units does not weaken the SI. Instead, it reflects the practical reality that established measurement conventions persist in particular fields. Scientists should distinguish between an SI unit, an accepted unit for use with the SI and a unit outside the SI.
SI units are realised physically through measurement procedures and technologies. Defining a unit does not mean that every laboratory possesses a direct physical object corresponding exactly to the definition. Instead, laboratories use instruments, reference standards, physical phenomena and calibration chains that realise the unit with stated uncertainty. Modern electrical metrology can use quantum phenomena to realise electrical quantities, while time metrology uses atomic transitions and highly stable frequency standards. Length can be realised through optical frequency and interferometric techniques. Mass can be realised using methods linked to the Planck constant, including the Kibble balance and other approaches. Practical realisations can improve over time without changing the fundamental SI definition.
SI units are essential to scientific communication because they allow numerical measurements to be interpreted consistently. They support experimental design, engineering calculations, calibration, international trade, manufacturing, healthcare and environmental monitoring. Their importance extends beyond choosing metres instead of feet or kilograms instead of pounds. The SI provides a coherent architecture connecting measurement definitions, physical constants, unit symbols, derived quantities, prefixes, calibration and traceability.
Correct SI usage also requires awareness of uncertainty and significant figures because a unit alone does not establish measurement quality. A value expressed in SI units can still be inaccurate, imprecise or highly uncertain. SI units provide the standard measurement framework, while the quality and interpretation of individual measurements depend on the instruments, methods, references, uncertainty evaluations and scientific reasoning used to obtain them.
