To compute the area of a square or rectangular area use AREA = Length X Width, to compute the amount of surface in a triangular area Area = 1/2 (Base X Height). With regard to square footage, when purchasing a new home measure it yourself or better yet have the selling agent/builder measure with you there watching and recording. For tax purposes, municipalities only count livable, heated space. Closets and hallways are included, but garages, attics, unfinished bonus rooms, and sun porches aren't. Nor, usually, are basementsâ€”though some localities include walk-out basements that are finished to the standard of the rest of the house. Because tax records aren't infallible, listing agents are supposed to measure the property themselves. But many just copy the information from tax records, and rely on boilerplate language in listing sheets and contracts that the "information is reliable but not guaranteed." Some also include space that a tax appraiser wouldn't, like a lavishly finished below-ground basement, and then use language that protects them from claims of deception, like "square footage on three levels." So before deciding how to proceed it's important to find out if the agent measured the house, what was measured and how it was done.