Length Conversion

Supported units

Every unit below is defined against the metre. To convert between any two, divide their factors: a mile is 1609.344 ÷ 0.3048 = 5280 feet.

Factors marked as exact are definitions rather than measurements: the international yard has been exactly 0.9144 m since the 1959 agreement, and the metre itself has been fixed by the speed of light since 1983 (SI Brochure). Note that the US survey foot was retired at the end of 2022; it differed from the international foot by two parts per million — irrelevant across a room, significant across a state.

Metric System

Angstrom (Å)

Equal to 0.1 nanometres, the angstrom measures distances at atomic scale: atomic radii, chemical bond lengths and the spacing between planes in a crystal. It is not an SI unit but remains in wide use in crystallography and spectroscopy.

Atomic and molecular measurements

Describes the size of atoms and the bond lengths between them.

X-ray crystallography

Used to express the resolution of X-ray diffraction patterns.

Wavelengths of light

Commonly used to measure the wavelengths of ultraviolet light.

Nanometer (nm)

One thousand-millionth of a metre. The nanometre is the working unit of semiconductor manufacturing, optics and nanotechnology, and is the conventional unit for the wavelength of visible light (roughly 380–750 nm).

Semiconductor manufacturing

Process nodes and feature sizes on integrated circuits.

Optics

Wavelengths of visible, ultraviolet and infrared light.

Materials science

Thickness of thin films and coatings.

Micrometer (µm)

One millionth of a metre, still often called the micron. It is the scale of biological cells, airborne particulates and machining tolerances.

Biology

Diameters of cells, bacteria and other microscopic structures.

Air quality

Particulate matter sizes such as PM2.5 and PM10.

Precision engineering

Surface roughness and machining tolerances.

Millimeter (mm)

One thousandth of a metre, and the default unit on engineering drawings in most of the world. Rainfall is also reported in millimetres, where one millimetre means one litre of water per square metre.

Technical drawing

The standard dimension unit in mechanical and architectural drawings.

Meteorology

Rainfall and snowfall depth.

Manufacturing

Component dimensions and fastener sizes.

Centimeter (cm)

One hundredth of a metre. The centimetre is the everyday metric unit for body measurements, paper and clothing sizes.

Everyday measurement

Height, clothing sizes and the dimensions of household objects.

Education

The unit most school rulers are marked in.

Medicine

Wound dimensions, and the size of anatomical features on imaging.

Decimeter (dm)

One tenth of a metre. Rarely used on its own, but the cubic decimetre is exactly one litre, which is why it survives in definitions of volume.

Volume definitions

One cubic decimetre is exactly one litre.

Hydrology

Occasionally used for water level changes.

Meter (m)

The SI base unit of length. Since 1983 it has been defined by fixing the speed of light at exactly 299 792 458 metres per second, so the metre is the distance light travels in vacuum in 1/299 792 458 of a second.

Science and engineering

The reference unit from which all other SI lengths derive.

Construction

Room dimensions, site surveys and building heights.

Sport

Track distances and field event measurements.

Kilometer (km)

One thousand metres, and the standard unit for road distances and geographic separation in most of the world.

Navigation

Road distances and travel planning.

Geography

Distances between settlements and the extent of regions.

Athletics

Race distances such as the 5 km and the 10 km.

Imperial System

Inch (in)

Defined as exactly 25.4 millimetres by the 1959 International Yard and Pound Agreement. Screen sizes, pipe diameters and tyre rim sizes are still quoted in inches almost everywhere.

Displays

Screen sizes are quoted as the diagonal in inches.

Construction and engineering

Pipe diameters, fastener sizes and timber dimensions.

Everyday measurement

Common in the United States for small dimensions.

Foot (ft)

Twelve inches, or exactly 0.3048 metres. Aviation still reports altitude in feet worldwide, and it remains the everyday unit of height in the United States and the United Kingdom.

Aviation

Altitude and flight levels are reported in feet internationally.

Construction

Room and building dimensions in the United States.

Everyday measurement

A person's height, usually with inches.

Yard (yd)

Three feet, or exactly 0.9144 metres. This is the definition on which the whole imperial system now rests: the 1959 agreement defined the yard metrically, and the foot and inch follow from it.

Sport

American football fields and some athletics distances.

Textiles

Fabric sold by the yard.

Landscaping

Bulk materials, though usually as cubic yards.

Mile (mi)

Exactly 1 760 yards, or 1 609.344 metres. The statute mile is the road distance unit of the United States and the United Kingdom.

Road distances

Signage and odometers in the US and UK.

Athletics

The mile remains a classic race distance.

Aviation and marine

Distinct from the nautical mile, which is longer.

Nautical Units

Nautical Mile (nmi)

Exactly 1 852 metres by international agreement, chosen to approximate one minute of latitude. That correspondence is what makes it useful: one nautical mile on a chart is one minute of arc along a meridian.

Marine navigation

Chart distances and vessel positions.

Aviation

Horizontal distances and separation minima.

Speed

One knot is one nautical mile per hour.

Fathom (fm)

Six feet, or exactly 1.8288 metres. Water depth on older charts is given in fathoms; modern charts increasingly use metres.

Marine charts

Sounding depths, especially on older or US charts.

Diving

Depth references in some traditional contexts.

Traditional Units

Link

One hundredth of a chain, or 7.92 inches. Links appear in historical land surveys and in property deeds that have never been restated in metric units.

Land surveying

Historical survey records and property boundaries.

Legal descriptions

Older deeds that still describe plots in chains and links.

Chain

Sixty-six feet, the length of Gunter's surveying chain. It ties the imperial system together neatly: ten square chains make exactly one acre, and a furlong is ten chains. A cricket pitch is one chain long.

Land surveying

The basis of the US Public Land Survey System.

Area calculation

Ten square chains make exactly one acre.

Cricket

The pitch is one chain between the wickets.

The four systems

Metric

Every metric unit is a power of ten away from the meter, which is what makes the system easy to work in: converting is a matter of moving the decimal point rather than remembering a conversion factor. The meter is defined by fixing the speed of light at exactly 299,792,458 meters per second, so it is tied to a constant of nature rather than to a physical object.

Imperial and US customary

Inches, feet, yards, and miles remain in everyday use in the United States and, for some purposes, the United Kingdom. The factors between them are irregular — 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile — which is precisely why a converter is useful. Since 1959 the inch has been defined as exactly 25.4 millimeters, so imperial and metric values now relate by an exact figure rather than an approximation.

Nautical

The nautical mile is 1,852 meters and corresponds to one minute of arc of latitude, which is what makes it natural for navigation: a degree of latitude is 60 nautical miles anywhere on Earth. The fathom, six feet, is the traditional unit of water depth. Note that a nautical mile is longer than a statute mile — roughly 1.15 times — so the two are not interchangeable on a chart.

Traditional surveying

The link and the chain come from Gunter's chain, a surveying instrument of 66 feet divided into 100 links. It looks arbitrary until you notice that ten square chains make exactly one acre, which is why land records in Britain, the United States, and much of the Commonwealth were kept in chains for centuries. Old deeds and property surveys are still written this way.

Worked examples

• A person 5 feet 10 inches tall— that is 70 inches, which converts to 177.8 centimeters exactly, since an inch is exactly 25.4 mm.
• A 26.2 mile marathon— about 42.195 kilometers, the official distance.
• A ship at 12 knots for 3 hours— 36 nautical miles, which is roughly 41.4 statute miles or 66.7 kilometers.
• A visible light wavelength of 550 nanometers— 5,500 angstroms, the unit conventionally used in spectroscopy.
• A plot of land 3 chains wide— 198 feet, or 66 yards.

Common uses

• Reading international specifications— component dimensions and technical drawings that arrive in the other system.
• Travel and navigation— converting road distances, altitudes, and chart distances.
• Construction and DIY— reconciling materials sold in metric sizes with plans drawn in feet and inches.
• Science and engineering— moving between micrometers, nanometers, and angstroms at small scales.
• Property and land records— interpreting older deeds written in chains and links.
• Sport— comparing race distances recorded in miles and kilometers.

Things worth knowing

• A mile is not a nautical mile.The difference is about 15%, which is more than enough to matter over any real distance.
• US survey feet differ slightly from international feet.The gap is roughly two parts per million — irrelevant for a room, significant across a state. The US officially retired the survey foot in 2022.
• Precision is not the same as accuracy.Converting a measurement rounded to the nearest foot into four-decimal-place centimeters does not make it more exact; the uncertainty of the original measurement carries through.
• Watch the abbreviations.A lowercase "m" is a meter, "mi" is a mile, and "mm" is a millimeter — three quantities that differ by many orders of magnitude.