The amount of three-dimensional space enclosed by a closed surface is expressed as a scalar quantity. For example, the space occupied or contained by a material (solid, liquid, gas, or plasma) or 3D form. Volume is frequently mathematically measured using the SI-derived unit, the cubic metre. The volume of a container is commonly believed to be its capacity; that is, the amount of fluid (gas or liquid) that the container can hold, rather than the amount of space that the container occupies. Volumes are also ascribed to three-dimensional mathematical forms. Volumes of some basic forms, such as regular, straight-edged, and circular shapes, may be determined quickly using arithmetic formulas.