In chemistry and fluid mechanics, the volume fraction φi is defined as the volume of a constituent Vi divided by the volume of all constituents of the mixture V prior to mixing: ϕ i = V i ∑ j V j {\displaystyle \phi _{i}={\frac {V_{i}}{\sum _{j}V_{j}}}} Being dimensionless, its unit is 1; it is expressed as a number, e.g., 0.18. Wikipedia