What term describes measuring components in a mixture based on volume?

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Measuring components by volume uses the amount of space each component occupies rather than its weight. This is captured by the term parts by volume, which expresses mixing ratios as volume units of one component to volume units of another. In automotive refinishing, you often mix coatings with precise volume ratios (for example, two parts by volume of color to one part by volume of reducer) to achieve the desired film thickness and curing behavior, especially since different materials can have different densities.

Mass fraction describes how much of each component weighs relative to total mass, which isn’t about volume. Volume percentage expresses the share of each component by volume as a percent of the total volume, which is similar in concept but the standard way to state volumetric mixing ratios in practice is using parts by volume. Parts by mass is explicitly a mass-based measure.