The volume fraction (also the volume part , fraction by volume ) is a dimensionless quantity equal to the ratio of the volume of a substance in a mixture to the sum of the volumes of the components before mixing  .
Usually denoted by the Greek letter. .
The concept applies mainly to gases and liquids. In the case of gases, the use of a volume fraction is convenient in that the volume fraction of gas in a mixture of chemically non-interacting gases is equal to its molar fraction in the same mixture, which is fairly accurately performed at not too high pressures and temperatures.
The volume fraction is determined by the formula:
- - volume fraction of component B;
- - volume of component B;
- - the sum of the volumes of all components.
When mixing some liquids, their total volume may vary relative to the sum of the volumes of the components, and therefore it is not always correct to replace the sum of the volumes of the components with the volume of the solution (mixture), for example, in mixtures of ethanol and water.
Sometimes, a similarly close value, usually called bulk concentration, is used. :
- - volume concentration of component B;
- - the volume of the pure component B before mixing;
- - the volume of the mixture.
Sometimes this value is also called the volume fraction, so it is recommended to specify the definitions of the values used to avoid discrepancies. For example, in GOST “Alcoholic products and raw materials for its production. The method of determining the volume fraction of ethyl alcohol "  , the volume fraction is understood as the quantity, in this article, called the volume concentration.
Units of Measurement and Designation
Both the volume fraction and the volume concentration are dimensionless quantities . For convenience, they are often expressed as a percentage , sometimes also using ppm and millionths . Sometimes, to emphasize that we are talking about a volume fraction (or volume concentration) they write “% vol.”, Or “vol.%”, - volume percent ( eng. % Vol. ). IUPAC does not recommend the use of such designations  ; instead, the value to which the indicated value refers should be explicitly indicated. For example, instead of "The concentration of oxygen in the air is 21% by volume." Should be written "The volume fraction of oxygen in the air is 21%."
- The concentration of solutions
- Beverage strength
- International Union of Pure and Applied Chemistry. IUPAC Gold Book - volume fraction, ϕ (English) . goldbook.iupac.org. The appeal date is December 10, 2018.
- GOST 32095-2013. Alcoholic products and raw materials for its production. Method for the determination of the volume fraction of ethyl alcohol.
- Quantities, Units and Symbols in Physical Chemistry (not available link) . UIPAC (2007). The date of circulation is December 31, 2018. Archived December 20, 2016.