How to Calculate Volume of a Rectangular Prism. A rectangular prism or solid is three-dimensional, and its volume is easy to calculate. You measure the volume of a rectangular solid in cubic units of measurement. Figure out the volume of a rectangular prism by following these few short and simple steps.
Find the length of the prism.
Find the width of the prism.
Find the height of the prism.
Multiply the 3 dimensions together. It does not matter the order in which you multiply the numbers.
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Write your answer in the proper cubic unit of measurement.
Make sure you use the same unit of measurement for each dimension. If you have the volume of the prism and need to find a missing dimension, work the problem backwards. Take the volume and divide it by each dimension given until you have the missing dimension. Break down complicated shapes into smaller parts. Determine the volume of smaller parts, and then combine them (by adding) into the volume of the entire shape.