Length units explained

Every imperial length here has been defined against the metre since 1959, so the conversions are exact rather than approximate. The oddities are historical: units that started as parts of a body or a day's ploughing, later pinned to a decimal.

Nanometre nm

1 nm = 1e-9 m Exact

A billionth of a metre — the scale of molecules and of the features etched onto a processor. Visible light has wavelengths of roughly 400 to 700 nm, which is why nothing smaller can be seen with an ordinary microscope no matter how good the lens.

nm to µm nm to mm nm to cm nm to m

Micrometre µm

1 µm = 0.000001 m Exact

A millionth of a metre, still widely called a micron. Human hair is 17 to 180 µm across, a red blood cell about 8 µm, and fine dust filters are rated by how small a particle in this range they can trap.

µm to nm µm to mm µm to cm µm to m

Millimetre mm

1 mm = 0.001 m Exact

The smallest unit most people handle directly. Engineering drawings and European timber and steel are dimensioned in millimetres precisely because there are no fractions involved — a thing is 12 mm or it is not.

mm to nm mm to µm mm to cm mm to m

Centimetre cm

1 cm = 0.01 m Exact

Convenient for body measurements and paper, and largely absent from engineering, which prefers millimetres and metres and skips the step between. Height is quoted in centimetres nearly everywhere outside the United States.

cm to nm cm to µm cm to mm cm to m

Metre m

1 m = 1 m Exact

The SI base unit. Originally meant to be a ten-millionth of the distance from the equator to the North Pole — an appealing idea slightly spoiled by the survey being a little off. It is now defined by the speed of light, which does not need surveying.

m to nm m to µm m to mm m to cm

Kilometre km

1 km = 1,000 m Exact

A thousand metres, and the unit of road distance in most of the world. A brisk walk covers one in about twelve minutes, which makes it a useful mental yardstick for whether somewhere is walkable.

km to nm km to µm km to mm km to cm

Inch in

1 in = 0.0254 m Exact

Exactly 25.4 mm since 1959. The name and the size both come from the thumb — as does the word in several European languages. Screen sizes, pipe diameters and tyre rim sizes are quoted in inches almost everywhere, including in countries that are otherwise entirely metric.

in to nm in to µm in to mm in to cm

Foot ft

1 ft = 0.3048 m Exact

Twelve inches, exactly 0.3048 m. Aviation is the reason it will not go away: altitudes are given in feet almost worldwide, so a change would mean retraining every pilot and controller at once. Also still standard for human height in the US and UK.

ft to nm ft to µm ft to mm ft to cm

Yard yd

1 yd = 0.9144 m Exact

Three feet, exactly 0.9144 m. Survives mainly in American football, cricket pitches, and fabric sold by the yard. Close enough to a metre that people mix them up, which matters over a distance: a hundred yards falls about 8.5 m short of a hundred metres.

yd to nm yd to µm yd to mm yd to cm

Mile mi

1 mi = 1,609.34 m Exact

1,760 yards, exactly 1.609344 km. The awkward 1,760 is Elizabethan: the Romans walked a thousand double paces, and later English law reconciled that with the furlong, which had to stay a whole number. Road distances in the US and UK are in miles; almost nowhere else is.

mi to nm mi to µm mi to mm mi to cm

Nautical mile nmi

1 nmi = 1,852 m Exact

Exactly 1,852 metres — one minute of latitude. That is the whole point: a navigator reading degrees off a chart can convert to distance without a table. Ships and aircraft use it worldwide, and speed in nautical miles per hour is the knot.

nmi to nm nmi to µm nmi to mm nmi to cm

Convert length

cm to in m to ft km to mi in to cm ft to m mi to km mm to in