| Ångströms (Å) | Micrometers (μm) |
|---|---|
| 1 Ångström | 0.0001 μm |
| 2 Ångströms | 0.0002 μm |
| 3 Ångströms | 0.0003 μm |
| 4 Ångströms | 0.0004 μm |
| 5 Ångströms | 0.0005 μm |
| 10 Ångströms | 0.001 μm |
| 20 Ångströms | 0.002 μm |
| 25 Ångströms | 0.0025 μm |
| 50 Ångströms | 0.005 μm |
| 100 Ångströms | 0.01 μm |
| Reference | Ångströms (Å) | Micrometers (μm) |
|---|---|---|
| A sheet of A4 paper (long side) | 2.97 × 109 Å | 297000 μm |
| Average adult human height | 1.7 × 1010 Å | 1700000 μm |
| A football pitch (length) | 1.05 × 1012 Å | 105000000 μm |
| A marathon | 4.2195 × 1014 Å | 4.2195 × 1010 μm |
| Height of Mount Everest | 8.849 × 1013 Å | 8.849 × 109 μm |
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 micrometre is one millionth of a metre, written µm using the Greek letter mu. It is also widely called the micron, a name the General Conference on Weights and Measures formally abolished in 1967 but which remains in everyday use across manufacturing and materials science.
The unit covers the scale of individual cells and fine particles. A human red blood cell is about 7 µm across. Bacteria typically measure 1 to 5 µm. Human hair ranges from roughly 17 to 180 µm in diameter, which is why hair is visible while cells are not. The limit of unaided human vision falls near 50 µm.
Air quality regulation relies on the micrometre. PM10 and PM2.5 refer to particulate matter smaller than 10 µm and 2.5 µm respectively. The distinction matters medically: larger particles are filtered by the nose and throat, while PM2.5 penetrates deep into the lungs and can enter the bloodstream.
Precision engineering works in the same range. Machining tolerances are commonly specified in micrometres, and surface roughness is quoted in the same unit. Bearing clearances, paint film thickness and the flatness of optical components are all measured this way. The measuring instrument called a micrometer, or micrometer screw gauge, is named for the precision it offers rather than for a fixed relationship to the unit.
Wavelengths in the infrared are usually given in micrometres rather than nanometres. Thermal imaging cameras typically operate between 8 and 14 µm, the band where objects at everyday temperatures emit most strongly.
Filtration is specified almost entirely in this unit. Water filters are rated by the smallest particle they retain, commonly between 0.2 and 50 µm, and a 0.2 µm filter is fine enough to remove most bacteria. Surgical masks and respirators are tested against particles in the same range. The N95 designation refers to a filter that captures at least 95 per cent of airborne particles at the hardest size to trap, around 0.3 µm.
Its older name still causes confusion. Until 1967 the unit was officially called the micron and written with a bare Greek mu, and the word remains common in industry even though the conference of that year removed it from the SI. Semiconductor manufacturing carried the name for a generation: chips were described by their process node in microns, falling from ten in the 1970s to below one by 1990, at which point the industry switched to nanometres and kept going. The modern node names no longer correspond to any physical dimension on the chip, but the earlier ones did, and they were quoted in exactly this unit.
One micrometre equals 1000 nanometres, 0.001 millimetres, or 10-6 metres.