What does attometre mean?
An attometre is a unit of length in the International System of Units equal to one quintillionth of a metre, or 10⁻¹⁸ m. It belongs to the family of metric prefixes built around powers of ten, sitting three orders of magnitude below the femtometre and well below any scale directly observable with light. Distances on this order arise mainly in nuclear and particle physics, where researchers describe features of atomic nuclei, precision measurements of fundamental constants, and the extraordinary sensitivity limits of instruments such as interferometers and gravitational-wave detectors. The name combines the SI prefix 'atto-', adopted in the twentieth century and derived from Danish 'atten', meaning eighteen, with 'metre'. Because it denotes such an extreme scale, the term appears almost entirely in formal scientific writing rather than everyday speech, though it serves as a vivid illustration of how far the metric system extends into the very small.
nounA unit of length in the metric system equal to one quintillionth (10⁻¹⁸) of a metre, formed from the SI prefix 'atto-'. It is used chiefly in physics to express extremely small distances, such as subnuclear scales.
- A metric unit of length equal to 10⁻¹⁸ metres, used for measuring subatomic-scale distances.
"The experiment probed length variations of just a few attometres across the atomic nucleus."
"The proton's charge radius is known to within a few attometres thanks to precision spectroscopy."
"Gravitational-wave detectors can now resolve displacements far smaller than an attometre at the mirror surfaces."
In scientific writing the plural follows normal English rules; values are often given as a number plus the symbol 'am'.
"The measurement was accurate to within five attometres."
An attometre is so small that if you scaled it up to the width of a human hair, the hair would stretch roughly the distance from Earth to beyond the Moon.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of attometre
The attometre derives from the SI prefix 'atto-', officially adopted in the early 1960s, combined with 'metre', the base unit of length. 'Atto-' comes from the Danish word 'atten', meaning eighteen, chosen because an attometre equals ten raised to the power of minus eighteen. The same prefix appears in cognate scientific units such as the attosecond, attogram, and attolitre. British spelling retains '-re', while American English typically uses 'attometer'.
Related word forms
How attometre is actually used
The spelling 'attometre' follows British convention; American usage typically prefers 'attometer'. The word is almost exclusively found in technical and scientific contexts and is rare outside physics literature.
Easily confused with attometre
A femtometre is 10⁻¹⁵ of a metre, a thousand times larger than an attometre (10⁻¹⁸); both are SI units but differ by three orders of magnitude.