Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance (solid, liquid, gas, or plasma) or 3D shape occupies or contains. Volume is often quantified numerically using the SI-derived unit, the cubic meter. The volume of a container is generally understood to be the capacity of the container, i.e., the amount of fluid (gas or liquid) that the container could hold, rather than the amount of space the container itself displaces. Three-dimensional mathematical shapes are also assigned volumes.