A regular solid's volume can be found using their respective formulas (cube, triangular prism, etc), while irregular solids can only be found by submerging the solid in water and measuring the amount the water rises.
For a regular its just Length x Width x Height. For an irregular u have to use displacement. Put it in a measured amount of water and then measure it again. The difference is the volume. (1 mL = 1 cM^3)