| Ångströms (Å) | Decimeters (dm) |
|---|---|
| 1 Ångström | 0.000000001 dm |
| 2 Ångströms | 0.000000002 dm |
| 3 Ångströms | 0.000000003 dm |
| 4 Ångströms | 0.000000004 dm |
| 5 Ångströms | 0.000000005 dm |
| 10 Ångströms | 0.00000001 dm |
| 20 Ångströms | 0.00000002 dm |
| 25 Ångströms | 0.000000025 dm |
| 50 Ångströms | 0.00000005 dm |
| 100 Ångströms | 0.0000001 dm |
| Reference | Ångströms (Å) | Decimeters (dm) |
|---|---|---|
| A sheet of A4 paper (long side) | 2.97 × 109 Å | 2.97 dm |
| Average adult human height | 1.7 × 1010 Å | 17 dm |
| A football pitch (length) | 1.05 × 1012 Å | 1050 dm |
| A marathon | 4.2195 × 1014 Å | 421950 dm |
| Height of Mount Everest | 8.849 × 1013 Å | 88490 dm |
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.
The decimetre is one tenth of a metre, written dm. It is a legitimate SI-derived unit formed with the prefix deci, but it is among the least used members of the metric ladder. Most people move directly from centimetres to metres and skip the intermediate step entirely.
The reason is practical rather than technical. A length of 3 dm is more naturally expressed as 30 cm or 0.3 m, and neither alternative requires the reader to pause. Units survive when they answer a question no neighbouring unit answers as well, and the decimetre rarely does.
One derived form is a striking exception. The cubic decimetre, dm3, is exactly one litre, because a cube measuring 10 cm on each side holds precisely that volume. This relationship is the foundation of the metric volume system and was deliberate: the litre was defined in 1795 as the volume of a cubic decimetre. The unit therefore appears constantly in chemistry, where concentration is expressed in moles per cubic decimetre, written mol/dm3. School and university chemistry courses use this notation routinely even though the same quantity could be written as moles per litre.
The square decimetre sees occasional use in specifying small areas, particularly in materials testing and in some European technical standards for coatings and surface treatment.
Outside these niches the decimetre appears mainly in teaching, where the full sequence of prefixes is demonstrated, and in tables of unit conversions. Some countries have used it historically for shoe sizing and for textile measurement.
Aquarium and tank capacities show the same relationship at work. A tank measuring five by three by four decimetres holds sixty cubic decimetres, which is sixty litres, and the arithmetic can be done without conversion factors. This is the practical advantage the metric system was designed to deliver, and the decimetre is the length at which volume in litres and length in whole units line up most neatly.
Scandinavia is the exception to its general neglect. Swedish, Norwegian and Danish speakers use the decimetre in ordinary conversation, giving the size of a fish, a shelf or a snowfall in decimetres where a French or German speaker would say tens of centimetres. Schools there teach it alongside the centimetre and the metre rather than skipping it, and rulers are marked accordingly. The habit is a reminder that which prefixes feel natural is a matter of custom rather than logic: the SI offers the whole ladder, and each language community has quietly settled on the rungs it finds comfortable.
One decimetre equals 10 centimetres, 100 millimetres, or 0.1 metres. It is approximately 3.937 inches, a little under four inches.