Any cross-section of a prism that is parallel to the base will be identical to the base. This means we can slice prisms up to help find their volume. For example, if we have a rectangular prism that is 3 units tall and has a base that is 4 units by 5 units, we can think of this as 3 layers, where each layer has cubic units. The volume of the figure is the number of cubic units that fill a three-dimensional region without any gaps or overlaps.

That means the volume of the original rectangular prism is , or 60, cubic units.

This works with any prism! If we have a prism with a height of 3 cm that has a base with an area of 20 cm², then the volume is cm³ regardless of the shape of the base. In general, the volume of a prism with height and area is

For example, these two prisms both have a volume of 100 cm³.