Distance is the measurement of length between two points and the numerical value of distance calculated in a specific unit and these units are known as unit of distance. For example, when we say the distance between Delhi and Mumbai is 1412Km then Km represents some unit and this is the unit of distance measured between two places. There are many units of distance in physics and used according to the need.

The term SI stands for Standard system of units and there are fundamental units that are defined in SI system and are used by all countries to avoid conflict while doing physical experiments and calculations. The SI unit of distance is known as the meter. The SI unit of distance meter is denoted by the letter ‘m’.

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**The smallest unit of length:**

The smallest numerical value we can assign for the measurement of length or say distance in any unit can be referred as the Smallest unit of length. In physics, the smallest unit of length is known as millimetre and denoted by ‘mm’. This smallest unit of length is related with the SI unit of distance as 1mm=10^{-3}m. In the case of the microscopic world the smallest unit of length is defined in the scale of atomic nuclei and the smallest unit of length in the microscopic world is called the Fermi metre denoted by ‘fm’ and its related to the SI unit of distance as 1Fm=10^{-15}m.

**Largest unit of length:**

The largest unit representation in which length can be measured is referred to as the largest unit of length. The largest unit of length is known as Kilometres denoted by ‘Km’ and its related to SI unit of distance as 1Km=1000m.

**CGS unit of Distance:**

The CGS stands for Centimetre Gram second and it’s also a system of units. The CGS unit of distance is known as centimetre and denoted by ‘cm’. it’s related to the SI unit of distance as 1cm=0.01m

**MKS unit of length:**

The MKS stands for Metre Kilogram second and it’s also a system of units. The MKS unit of distance is known as metre and denoted by ‘m’. and This unit as mentioned is the SI unit of distance.

**Unit of distance in Astronomy:**

In astronomical physics, the distance between objects in space is very large, so a large unit is needed to represent such distance and thus distances measured in astronomical physics are referred to as Units of distance in astronomy. The unit of distance in astronomy is called the Light year. One light-year is the distance travelled by the light with the speed of light in free space in one year. The unit of distance in astronomy ‘light year’ is related with SI unit of distance as 1Light year=9.46×10^{15}m.

**Units of length Smallest to largest:**

The various units of length from smallest to largest are mentioned in the table given below in increasing order of units.

Name of the unit | Symbol of the unit | Relationship with SI unit of distance |

millimetre | mm | 1mm=0.001m |

centimetre | cm | 1cm=0.01m |

metre | m | It’s the SI unit of distance. |

Kilometre | Km | 1Km=1000m |

Apart from these, the smallest unit of length is generally Fermi meter while the largest unit of length is Light year in terms of microscopic and macroscopic measurement of distances.

**Definition of 1 metre:**

In an international system of units, one metre is defined as the actual length covered by the light in free space with the speed of light c=3×10^{8}ms^{-1} in the time interval of 1/299792458 of a second. and this distance is called one metre.

**Distance Unit chart:**

Some popular units in which distances are measured along with their conversions in the standard unit of distance are given below in the distance unit chart.

Name of the unit | Relation with standard unit |

Inches | 1in=0.0254m |

Yards | 1yard=0.9144m |

Kilometre | 1Km=1000m |

Centimetre | 1cm=0.01m |

Feet | 1ft=0.3048m |

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