# Length

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### Distance

**distancesproximitydepth**

Length is a measure of distance. In the International System of Quantities, length is any quantity with dimension distance.

In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over").

### International System of Quantities

**base quantitiesISQbase quantity**

In the International System of Quantities, length is any quantity with dimension distance.

The International System of Quantities (ISQ) is a system based on seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

### System of measurement

**system of unitssystems of measurementmeasurement system**

In most systems of measurement, the unit of length is a base unit, from which other units are derived.

In most systems, length (distance), mass, and time are base quantities.

### Geometry

**geometricgeometricalgeometries**

In geometry, length is the most extended dimension of an object.

Geometry arose independently in a number of early cultures as a practical way for dealing with lengths, areas, and volumes.

### Area

**surface areaareassize**

Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed).

It is the two-dimensional analog of the length of a curve (a one-dimensional concept) or the volume of a solid (a three-dimensional concept).

### Unit of length

**lengthunits of lengthL**

Many units of length have been used throughout history and in different civilisations and cultures: until quite recently it was common for length units to be differently defined even between neighbouring towns and cities.

A unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension.

### Height

**tallhhigh**

Length may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length.

In elementary models of space, height may indicate the third dimension, the other two being length and width.

### Cubit

**cubitsroyal cubitcubit arm**

For instance one of the oldest units of length measurement used in the ancient world was the cubit, which was the length of the arm from the tip of the finger to the elbow.

The unit was based on the forearm length from the tip of the middle finger to the bottom of the elbow.

### Unit of measurement

**unitunits of measurementunits**

There are several units that are used to measure length.

For example, a length is a physical quantity.

### Volume

**volumetriccapacityvolumes**

Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed).

Any unit of length gives a corresponding unit of volume: the volume of a cube whose sides have the given length.

### SI base unit

**base unitbase unitsbase**

In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light.

The SI base units and their physical quantities are the meter for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.

### International System of Units

**SISI unitsSI unit**

In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light.

### Kilometre

**kmkilometerkilometers**

The centimetre and the kilometre, derived from the metre, are also commonly used units.

The kilometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: km; or ) or kilometer (American spelling) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for 1000).

### Centimetre

**cmcentimetercentimeters**

The centimetre and the kilometre, derived from the metre, are also commonly used units.

A centimetre (international spelling as used by the International Bureau of Weights and Measures; symbol cm) or centimeter (American spelling) is a unit of length in the metric system, equal to one hundredth of a metre, centi being the SI prefix for a factor of 1⁄100.

### Inch

**ininchesinternational inch**

In U.S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.

The inch (abbreviation: in or ″) is a unit of length in the (British) imperial and United States customary systems of measurement.

### Foot (unit)

**feetftfoot**

In U.S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.

The foot ( pl. feet; abbreviation: ft; symbol: ′, the prime symbol) is a unit of length in the imperial and US customary systems of measurement.

### Measurement

**measuremeasuringmeasured**

There are several units that are used to measure length.

### Orders of magnitude (length)

**mmnmgigameter**

Orders of magnitude (length)

The following are examples of orders of magnitude for different lengths.

### Arc length

**rectifiable curvearclengthlength**

Arc length

Since it is straightforward to calculate the length of each linear segment (using the Pythagorean theorem in Euclidean space, for example), the total length of the approximation can be found by summing the lengths of each linear segment; that approximation is known as the (cumulative) chordal distance.

### Reciprocal length

**cm −1 inverse centimeterinverse length**

Reciprocal length

As the reciprocal of length, common units used for this measurement include the reciprocal metre or inverse metre (m −1 ), the reciprocal centimetre or inverse centimetre (cm −1 ), and, in optics, the dioptre.

### Metre

**metermmetres**

In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light.

) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in some metric systems, including the International System of Units (SI).

### Metric system

**metricmetric unitsmetric unit**

### Astronomical unit

**AUastronomical unitsAUs**

Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the astronomical unit, the light-year, and the parsec.

### Parsec

**Mpcpckpc**

Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the astronomical unit, the light-year, and the parsec.

### Base unit (measurement)

**base unitbase unitsfundamental quantity**

In most systems of measurement, the unit of length is a base unit, from which other units are derived.