The Metre-Kilogram-Second or MKS system of units is a physical system of units that expresses any given measurement using the three fundamental units of the Metre, Kilogramme, and/or Second. The MKS system, widely known as the Metric System, has succeeded the CGS system.

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The **MKS system of units** is a physical system of units that expresses any given measurement using base units of the metre, kilogram, and/or second (MKS).

Historically the use of the MKS system of units succeeded the centimetre–gram–second system of units (CGS) in commerce and engineering, (1889). The metre and kilogram system served as the basis for the development of the International System of Units, which now serves as the international standard. Because of this, the standards of the CGS system were gradually replaced with metric standards incorporated from the MKS system.

An advantage of MKS units is that since the derived units (like joules) are based on MKS units, using MKS units naturally gives answers in the appropriate SI unit. For example, the kinetic energy of an object is defined by 1/2 × mass × velocity^{2}. If the calculation is done in MKS units of kilogram and meters per second then the result is in joules, the SI unit for energy. If the same calculation is done using CGS units the answer is in ergs ( = 1 erg^{−7} joules 10).

The exact list of units used in the MKS system changed over time. It incorporated base units other than the metre, kilogram, and second in addition to derived units. An incomplete list of the base and derived units appears below. Since the MKS system of units never had a governing body to rule on a standard definition, the list of units depended on different conventions at different times.

- Cycle (This dimensionless quantity became synonymous with the term "cycle per second" as an abbreviation. This circumstance confused the exact definition of the term cycle. Therefore, the phrase "cycle per metre" became ill-defined. The cycle did not become an SI unit.)
- Cycle per second
- Cycle per metre (This measure of wavenumber became ill-defined due to the abbreviation of "cycle per second" as "cycle".)

In 1901, Giovanni Giorgi proposed to the Associazione elettrotecnica italiana (AEI) that this system, extended with a fourth unit to be taken from the units of electromagnetism, be used as an international system.

This system was strongly promoted by electrical engineer George A. Campbell.