What does octane mean?
Octane most commonly denotes a measure of how resistant petrol is to knocking, an unwanted combustion phenomenon that can cause a spark-ignition engine to lose efficiency or sustain damage. A fuel's octane rating or number indicates its resistance under compression; higher values generally allow greater compression before knocking occurs. The term also names a particular colourless, flammable hydrocarbon containing eight carbon atoms and two hydrogen atoms, a substance found in petroleum and used as a constituent of fuel. The numerical fuel rating is connected to this compound through standardised tests comparing a fuel's knock resistance with reference fuels of known octane values. In British English, “octane rating” is especially common, while “octane number” is frequent in American English, although both expressions occur on both sides of the Atlantic. Octane is therefore both the name of a specific molecule and a practical measure used to describe the performance and suitability of road fuel.
A numerical measure of the resistance of petrol or another spark-ignition fuel to knocking.
"Changing to a higher-octane fuel reduced the knocking heard under acceleration."
Also called an octane number; the numerical rating is commonly stated as 87, 89, 91, 95, 98 or 100 in modern vehicle fuel.
A colourless, flammable alkane hydrocarbon with eight carbon atoms and the molecular formula C8H18.
"Octane is one of several hydrocarbons present in refined petroleum."
This technical chemical sense is less common in everyday usage than the fuel-rating sense.
The plural is uncommon in general use but may refer to the eight-carbon alkanes or their isomers in technical contexts.
"The researchers compared the properties of several octanes under controlled conditions."
Octane is named for the number eight because the original compound has eight carbon atoms in each molecule.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does octane come from?
The word combines Greek oktō, meaning “eight,” with the chemical suffix -ane, which is used in the names of saturated hydrocarbons. Its name reflects the eight-carbon structure of the compound C8H18. The element meaning “eight” also appears in related words such as octave, octet and October, all of which ultimately derive from the same ancient numerical root.
Related word forms
How octane is actually used
In everyday British and American English, “octane” usually refers to the fuel rating rather than the specific hydrocarbon. The fuel measure is often called the octane number in American English and the octane rating in British English, though both terms are widely understood.
Easily confused with octane
Octane measures a petrol or gasoline engine's resistance to knocking, whereas cetane measures the ignition quality of diesel fuel.