Unit category

Length Units and Conversions

Learn about metric and imperial length units and convert between meters, kilometers, centimeters, millimeters, inches, yards, miles and feet.

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0.001 km
1 m = 0.001 km

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Metric

US / Imperial

Traditional and regional

Scientific

  • Astronomical Unit6.68459E-12 AU
  • Light-year1.057E-16 ly
  • Parsec3.24078E-17 pc
  • Angstrom10,000,000,000 Å

Popular conversions

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Unit guides

24 units

Detailed guide to Length

Length is a physical quantity used to describe the distance between two points or the size of an object along a particular direction. It is one of the fundamental quantities used in science, engineering and everyday measurement.

The meter is the base unit of length in the International System of Units. Other metric length units are connected to the meter through decimal prefixes, while imperial and United States customary units use separately defined conversion relationships.

Physical quantity
Length
SI base unit
Meter
SI symbol
m
Dimension symbol
L
Modern definition
Distance traveled by light in vacuum during 1/299,792,458 of a second

Metric and imperial length in practice

Metric units are standard in most countries, while inches, feet, yards and miles remain common in US and UK-facing drawings, road distances and product specifications.

For construction, manufacturing or fit-critical work, convert the stated unit rather than assuming that an unlabelled value is metric.

What is length?

Length describes a one-dimensional extent. Depending on the direction being measured, it may represent distance, height, width, depth, thickness or diameter.

In dimensional analysis, length is represented by the symbol L. Many derived physical quantities, including area, volume, speed, acceleration and pressure, contain a length dimension.

The meter as the SI base unit

The meter is one of the seven base units of the International System of Units. It provides the reference from which metric length units and many derived units are constructed.

The modern meter is linked to the fixed value of the speed of light in vacuum. This makes the definition reproducible without depending on a physical measuring bar or a particular location on Earth.

Metric length units

Metric length units use prefixes that represent powers of ten. One kilometer equals 1,000 meters, one centimeter equals 0.01 meters and one millimeter equals 0.001 meters.

For extremely small measurements, units such as the micrometer and nanometer are used. Large geographical and road distances are commonly expressed in kilometers.

Imperial and US customary length units

The foot, mile, inch and yard are widely recognized non-metric length units. They are used primarily in the United States and in selected applications in the United Kingdom and other countries.

One international foot is exactly 0.3048 meters, while one international mile is exactly 1,609.344 meters. These exact definitions provide reliable conversion factors.

Nautical and aviation length units

Distances at sea and in the air are usually expressed in nautical miles rather than kilometers or statute miles. One nautical mile is defined as exactly 1,852 meters.

The nautical mile developed from a historical link between distance and the Earth's geographic coordinates: one minute of latitude corresponds to approximately one nautical mile. The unit of speed called the knot means one nautical mile per hour.

How are length units converted?

A length conversion changes the numerical value and unit while preserving the physical distance being represented. The original value is multiplied by the conversion factor between the two units.

Metric conversions often involve powers of ten. Conversions between metric and imperial systems require defined factors such as 2.54 centimeters per inch or 1,609.344 meters per mile.

Length measurement and uncertainty

Length may be measured using rulers, tape measures, calipers, micrometers, laser distance meters and optical instruments. The appropriate instrument depends on the size of the object and the required precision.

Every physical measurement has uncertainty. Instrument resolution, calibration, temperature, measurement technique and environmental conditions can all affect the reported value.

Length unit comparison table

UnitSymbolMeter equivalentSystemCommon use
Nanometernm0.000000001 mSI/metricLight wavelength and nanotechnology
Micrometerµm0.000001 mSI/metricCells, particles and precision manufacturing
Millimetermm0.001 mSI/metricTechnical drawings and small measurements
Centimetercm0.01 mSI/metricEveryday object measurements
Decimeterdm0.1 mSI/metricEducation and some volume relationships
Meterm1 mSIBase length measurements
Kilometerkm1,000 mSI/metricRoad and geographic distances
Inchin0.0254 mImperial/USScreens, pipes and technical measurements
Footft0.3048 mImperial/USHeight, construction and aviation
Yardyd0.9144 mImperial/USSports fields and distance measurements
Milemi1,609.344 mImperial/USRoad distances
Nautical milenmi1,852 mNauticalMaritime and aviation navigation

Length units

UnitSymbolInformation
MetermView unit guide
KilometerkmView unit guide
CentimetercmView unit guide
MillimetermmView unit guide
MilemiView unit guide
FootftView unit guide
InchinView unit guide
YardydView unit guide
DecimeterdmView unit guide
MicrometerµmView unit guide
NanometernmView unit guide
PicometerpmView unit guide
FurlongfurView unit guide
Nautical milenmiView unit guide
Astronomical UnitAUView unit guide
Light-yearlyView unit guide
ParsecpcView unit guide
AngstromÅView unit guide
FathomftmView unit guide
ArshinarshinView unit guide
EndazeendazeView unit guide
Byzantine FootpusView unit guide
Byzantine FathomorgyiaView unit guide
CigcigView unit guide

Sources

  1. BIPM: The International System of Units (SI Brochure)
  2. NIST: SI Units – Length
  3. NIST: Guide for the Use of the International System of Units

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