SI units are most widely used units of measurement belong to an international system of units which is also known as SI or Metric System. SI comprises of three main categories which are prefixes, base units and derived units.

SI base units are seven which are defined as;

**Length (Meter)**

The meter is the length of the path which light covers in an interval of 1/299,792,458 of a second.

**Mass (Kilogram)**

The mass unit is kilogram (kg) which is equal to the international prototype of the kilogram.

**Time (second)**

The second is the span of 9,192,631,770 intervals of the radiation which relates the motion between two hyperfine levels of the ground condition of cesium particle.

**Luminous Intensity (Candela)**

The candela is the luminous intensity, in a direction in which a source emits or releases radiation of frequency 540*10^{12} Hz which has 1/683 watt/steradian radiant intensity in that direction.

**Temperature (Kelvin)**

The unit of temperature is Kelvin, which is actually 1/273.16 th fraction of the temperature of the triple point of water.

**Electric Current (Ampere)**

The ampere is a constant current which produces a force of 2*10^{-7 }N/m of length between two parallel conductors which are 1m apart in a vacuum and have infinite length.

** ****Amount of Substance (mol)**

The mol is the amount of substance which comprises of elementary entities equivalent to atoms in 0.12 kg of carbon.

**Derived Units**

Derived units are, basically, derived from base units using algebraic relations which actually link different base quantities in an organized fashion. Some of the derived units are given in the table below.

Quantity | Symbol | Unit |
---|---|---|

Frequency | Hz | Hertz |

Force | N | Newton |

Energy | J | Joule |

Power | W | Watt |

Electric Charge | C | Coulomb |

Voltage | V | Volt |

Electric Resistance | Ω | Ohm |

Magnetic Field | T | Tesla |

Capacitance | F | Farad |

Inductance | H | Henry |

**SI Units Prefixes**

Metric prefixes and the letter symbols for the various multiples and submultiples of 10 are listed in Table below, with those most commonly used with electrical units. The prefixes are employed to simplify the writing of very large and very small quantities. Thus, 1000 Ω can be expressed as 1kohm or 1k Ω. Here kilo is the prefix that represents 1000 and k is the symbol.

Power | Prefix | Symbol |
---|---|---|

\[{{10}^{18}}\] | Exa | E |

\[{{10}^{15}}\] | Peta | P |

\[{{10}^{12}}\] | Tera | T |

\[{{10}^{9}}\] | Giga | G |

\[{{10}^{6}}\] | Mega | M |

\[{{10}^{3}}\] | Kilo | K |

\[{{10}^{2}}\] | Hecto | h |

\[10\] | Deka | da |

\[{{10}^{-1}}\] | Deci | d |

\[{{10}^{-2}}\] | Centi | c |

\[{{10}^{-3}}\] | Milli | m |

\[{{10}^{-6}}\] | Micro | µ |

\[{{10}^{-9}}\] | Nano | n |

\[{{10}^{-12}}\] | Pico | p |

\[{{10}^{-15}}\] | Femto | f |

\[{{10}^{-18}}\] | Atto | a |

SI units will be revised in near future and four of the base units such as mol, kilogram, ampere, kelvin will be represented in terms of different constants which are Boltzmann constant, Avogadro constant, Planck constant and elementary charge.

** ****SI units or Matric System Advantages**

- They are easy to use in real world systems.
- We can derive so many other units from base units just by multiplication and division.
- It is an internationally accepted system of units because of flexible and convenient use.