Answer: Inside (depending on your definition).
Step-by-step explanation: Finding the volume of a solid means measuring what space that solid takes up. Volume is a measure of how much matter an object is made up of. Technically, finding the volume of a solid does not mean finding the "inside" or "outside" of a solid. If you are referring to the surface area of a solid as the outside, then the answer to your question would be the inside of the solid.