The international systems of units (abbreviated as SI) is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world.

There are several SI units used in physics that are used to express the different quantities. The quantities can be classified into two groups i.e. BASE UNITS and DERIVED UNITS.

7 Base Units and 22 Derived Units with special names and symbols may be used in combination to express other derived units.

**There are 7 SI base units:**

**Unit of length, meter (m):**Meter is the SI unit of length and is defined by taking the fixed value of the speed of light in vacuum.**Unit of mass, kilogram (kg):**Kilogram is the SI unit of mass and is defined by taking the fixed value of the planck constant.**Unit of time, second (s):**Second is the SI unit of time and is defined by taking the fixed value of Cesium frequency.**Unit of electric current, ampere (A):**Ampere is the SI unit of electric current and is defined by taking the fixed value of the elementary charge.**Unit of thermodynamic temperature, kelvin (K):**Kelvin is the SI unit of thermodynamic temperature and is defined by taking the fixed value of Boltzmann constant.**Unit of the amount of substance, mole (mol):**Mole is the SI unit of amount of substance and is defined by the fixed value of Avogadro constant. One mole contains 6.023 x 10^{23}elementary entities.**Unit of luminous intensity, candela (cd):**Candela is the SI unit of luminous intensity and is defined by the fixed value of the luminous efficacy.

S.No | Name of the Quantity | SI Unit | SI Unit Symbol |
---|---|---|---|

1 | Length | Meter | m |

2 | Mass | Kilogram | kg |

3 | Time | Second | s |

4 | Electric Current | Ampere | A |

5 | Thermodynamic Temperature | Kelvin | K |

6 | Amount of Substance | Mole | mol |

7 | Luminous Intensity | Candela | cd |

**The MKS System**

This system of units has the meter, kilogram and second as its base units. So the MKS is the static core, and the SI is the evolving, growing system of metric measurement. Metres and Kilograms were introduced after the French Revolution around 1790. Seconds were added by the 1830s, building the MKS system.

More units came later. By 1948, a need was seen to develop a clear and consistent set of terms for measurement, and this led to 12 years of work, ending with the release of the first version of the SI in 1960s.

**The CGS System**

The centimeter-gram-second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.

**The FPS System**

In FPS system, the units of length, mass and time are the foot, pound and second(s).