| Kilometers (km) | Ångströms (Å) |
|---|---|
| 1 Kilometer | 10000000000000 Å |
| 2 Kilometers | 20000000000000 Å |
| 3 Kilometers | 30000000000000 Å |
| 4 Kilometers | 40000000000000 Å |
| 5 Kilometers | 50000000000000 Å |
| 10 Kilometers | 100000000000000 Å |
| 20 Kilometers | 200000000000000 Å |
| 25 Kilometers | 250000000000000 Å |
| 50 Kilometers | 500000000000000 Å |
| 100 Kilometers | 1 × 1015 Å |
| Reference | Kilometers (km) | Ångströms (Å) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.000297 km | 2.97 × 109 Å |
| Average adult human height | 0.0017 km | 1.7 × 1010 Å |
| A football pitch (length) | 0.105 km | 1.05 × 1012 Å |
| A marathon | 42.195 km | 4.2195 × 1014 Å |
| Height of Mount Everest | 8.849 km | 8.849 × 1013 Å |
The kilometre is one thousand metres, written km. It is the standard unit for road distance, geographic separation and travel across almost the entire world, and after the metre itself it is the most widely used SI length unit.
Road signs, speed limits, vehicle odometers and navigation systems use it in every country except the United States, Liberia and Myanmar, where miles remain in place. The United Kingdom occupies a middle position: road signs and speed limits are in miles, while most other official measurement is metric. Running and cycling events are almost universally set in kilometres, with the marathon a notable exception at 42.195 km, a distance fixed by the route used at the 1908 London Olympics rather than by any round figure.
The unit suits the human scale of travel. A brisk walk covers about five kilometres in an hour. Urban journeys are typically a few kilometres, and national distances run to hundreds or thousands. Below one kilometre metres are more natural, and above a few thousand many people switch to comparing flight times instead.
Pronunciation is contested. The stress pattern KIL-o-metre follows the pattern of other SI prefixed units such as kilogram and kilowatt, where the prefix is unstressed. The alternative, ki-LOM-eter, is nonetheless dominant in North America and common in Australia and New Zealand, and both are recorded as standard by major dictionaries.
The kilometre also anchors two other units. A square kilometre is one million square metres, or 100 hectares, and is the usual measure for the area of towns, lakes and protected land. Speed is expressed as kilometres per hour, written km/h.
Astronomy uses the kilometre for objects within the solar system, where larger units would be unwieldy. The Moon averages 384,400 km from Earth and the Sun about 149.6 million km. Beyond that distance astronomers switch to astronomical units, light years and parsecs, because the kilometre figures become too long to read. Orbital velocities are given in kilometres per second: the International Space Station travels at roughly 7.66 km/s.
One kilometre equals 1000 metres or 100,000 centimetres. It is approximately 0.621371 miles, so a five-kilometre race is a little over three miles.
The ångström is a unit of length equal to one ten-billionth of a metre, or 0.1 nanometres. It takes its name from Anders Jonas Ångström, the Swedish physicist who used it in his 1868 map of the solar spectrum. The symbol is Å, a letter borrowed from the Swedish alphabet.
The ångström survives because it matches the scale of atoms. A hydrogen atom has a radius of about 0.5 Å. A carbon-carbon single bond measures roughly 1.5 Å. Expressing these figures in nanometres produces awkward decimals, so crystallographers, spectroscopists and structural biologists continue to prefer the older unit. Protein structures deposited in public databases are still described by their resolution in ångströms, and a structure resolved to better than 2 Å is considered high quality.
Wavelengths of visible light also fall in a convenient range. Red light sits near 7000 Å and violet near 4000 Å. X-ray wavelengths cluster around 1 Å, which is precisely why X-ray diffraction reveals atomic spacing: the probe and the target are the same size.
The ångström is not part of the International System of Units. The BIPM lists it among units that are accepted for use with SI but discourages new applications, preferring the nanometre or picometre. That guidance has had limited effect in the fields where the unit is entrenched. Semiconductor manufacturing offers a clear illustration. Process nodes were named in nanometres for decades, but as features shrank the industry began quoting gate oxide thicknesses in ångströms, and Intel named a generation of its technology the Angstrom era.
Reading older scientific literature requires care. Before the ångström was tied to the metre it was defined against a specific spectral line of cadmium, and figures published in the early twentieth century may differ slightly from modern values. The International Astronomical Union adopted that spectroscopic definition in 1907, and it stood until the metre itself was redefined against krypton in 1960. The discrepancy is small, but it is real, and it matters when comparing historical spectral measurements against current ones.
Converting is straightforward. One ångström equals 10-10 metres, 0.1 nanometres, or 100 picometres. Ten ångströms make a nanometre.