Volume fraction

In chemistry and fluid mechanics, the volume fraction

is defined as the volume of a constituent Vi divided by the volume of all constituents of the mixture V prior to mixing:[1] Being dimensionless, its unit is 1; it is expressed as a number, e.g., 0.18.

It is the same concept as volume percent (vol%) except that the latter is expressed with a denominator of 100, e.g., 18%.

It has as a denominator the volume of the mixture itself, as usual for expressions of concentration,[2] rather than the total of all the individual components’ volumes prior to mixing: Volume percent is usually used when the solution is made by mixing two fluids, such as liquids or gases.

In the case of a mixture of ethanol and water, which are miscible in all proportions, the designation of solvent and solute is arbitrary.