| Reference | Meters (m) | Ångströms (Å) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.297 m | 2.97 × 109 Å |
| Average adult human height | 1.7 m | 1.7 × 1010 Å |
| A football pitch (length) | 105 m | 1.05 × 1012 Å |
| A marathon | 42195 m | 4.2195 × 1014 Å |
| Height of Mount Everest | 8849 m | 8.849 × 1013 Å |
The metre is the base unit of length in the International System of Units, written m. Every other SI length unit is defined as a multiple or fraction of it, and units of area, volume, speed and many others are built from it in turn.
Its definition has been revised four times, each revision replacing a physical object with something more reproducible. The French Academy of Sciences proposed in 1791 that the metre should be one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. Surveying that arc took six years and produced a small error, since the Earth is not a perfect sphere. In 1889 the definition moved to a platinum-iridium bar held at Sèvres, near Paris, with copies distributed to signatory nations. In 1960 it was redefined as 1,650,763.73 wavelengths of orange-red light emitted by krypton-86.
The current definition dates from 1983 and is unusual in that it fixes a different quantity. The speed of light in vacuum is defined as exactly 299,792,458 metres per second, and the metre follows as the distance light travels in 1/299,792,458 of a second. Light speed is therefore no longer measured; it is a defined constant, and improvements in measurement now refine the metre rather than the speed.
The unit is used worldwide for building dimensions, athletics tracks, swimming pools, water depth, fabric and rope. Only a small number of countries retain imperial units for general purposes, and even there the metre dominates science and medicine.
Everyday reference points help fix the scale. A standard interior door is roughly two metres tall. A single stride for an adult is close to three quarters of a metre, which is why pacing is a workable rough measure of distance. The width of a single traffic lane is between three and three and a half metres in most countries.
One metre equals 100 centimetres, 1000 millimetres, or 0.001 kilometres. It is approximately 3.28084 feet, or 39.3701 inches.
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.