Ångströms and nanometres — the atomic-scale units
1 Å = 0.1 nm — a rounding-friendly relationship that lets two whole scientific disciplines keep their favourite units without arguing.
The ångström is not an SI unit, but the Bureau International removed it from the SI supplement in 2019 without asking chemistry. X-ray crystallographers still report unit-cell parameters in Å, protein data bank entries still list bond lengths in Å, and no one who works with electron density is switching soon.
The exact factors
1 Å = 0.1 nm = 100 pm = 10⁻¹⁰ m (exact) > Visible light: 380–780 nm (3,800–7,800 Å) > Typical covalent bond: 1.0–2.5 Å (100–250 pm) > Hydrogen atom radius: 0.53 Å
Why chemistry stayed on the ångström
Most covalent bond lengths land between 1 and 2 Å. Reporting them as 0.1–0.2 nm loses a decimal digit of feel — every number becomes 0-point-something. The picometre (100 pm = 1 Å) works too, but a century of literature is written in Å, and re-tabulating it is nobody's job.
Why physics moved to nanometres
Optical wavelengths are 400–800 nm — comfortable three-digit integers. Semiconductor process nodes (5 nm, 3 nm, 2 nm) fit the same range. Once the industry standardised on nm, the ångström stopped appearing in modern optical and semiconductor papers.
Worked example — water
The O–H bond in water is 0.958 Å = 95.8 pm = 0.0958 nm. The H–O–H angle is 104.45°. Molecular dynamics codes usually take Å; DFT codes usually take bohr (0.529 Å); optical codes take nm. Same molecule, three unit conventions, and every input file needs the conversion right.
The five-second recap
1 Å = 0.1 nm = 100 pm. Chemistry uses Å, physics uses nm, quantum chemistry uses bohr. Always check the header of the file before you multiply.
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FAQ
Is the ångström still an SI unit?+
No — it is 'accepted for use with the SI' but not part of it. In practice, chemistry journals accept it without comment.
What is a bohr?+
The atomic unit of length: 0.529177 Å. Used almost exclusively in quantum chemistry software input files.
Why do semiconductor nodes not match feature size any more?+
The '3 nm' node is a marketing name, not a physical feature. Actual transistor gate pitch is closer to 45 nm.
This article was written by The Turbo Unit Converter engineering desk (Physics & chemistry) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.