A rectangular prism has three dimensions: length (l), width (w), and height (h).
A rectangular prism has volume formula V = lwh. So, for example, if a rectangular prism has dimensions l = 12 in., w = 4 in., and h = 3 in., then V = 12 in. x 4 in. x 3 in. = 144 in.³
Another way to think about the volume formula is as follows: V = l w h = (l w) h.
Since l w is the area of the prism's base, you can think of the volume formula as the area of the base times the height, or V = Bh.