Identify three perpendicular dimensions
A rectangular prism has rectangular faces. Measure its length, width and height; a diagonal is not a substitute for any of these.
Multiply the measurements
For a box measuring 5 cm × 4 cm × 3 cm, volume = 5 × 4 × 3 = 60 cm³.
Distinguish capacity from exterior size
For a container’s usable capacity, use its internal measurements. Thick walls and a raised base make the internal volume smaller than the volume calculated from its exterior dimensions.