A prism is a 3-dimensional object, so this will inform our answer, but that's as far as it goes; we might as well say "a rectangular box", because the fact that it's a prism doesn't affect the way we measure volume.

Volume for a rectangular, 3-dimensional object is always Length x Width x Height.

So, if the prism has sides that equal (for example) 10m, 5m and 3m, the volume will be

10 x 5 x 3 = `150m^3`

remember that your units will be *cubed* - written using the superscript "3"

Using the same picture as one of the prior responses, we can think of solving the volume of a prism as finding the 2-dimensional "base" and multiplying by its height, or unused measurement. In the example in the picture, if you choose the base to be the darkest shaded spot and isolate it, the rectangle is a 2 X 4 inch rectangle. The area of it is then 8 inches squared. By taking this base and multiplying it by the height, or 13 inches, you find that the volume is then 104 inches cubed. It is cubed because you are using 3 units of measurement, (length X width) X height.

length x width x height is the formula for finding a rectangular prism, mainly because it is a 3d shape.

To find the volume for a rectangular prism the formula is

length x Width x Height

Rectangular prism is a 3 dimensional shape