If you need the sq/ft then measure length x width in feet; example- a wall is 10ft long with 8ft ceiling height= 80 sq/ft. Also take into account the roughness (texture) of the surface, its color you need to cover (could take 2 or more coats), the application method and the quality of the paint used. All of these things affect the cost.
Dave's way is how I do it. After years of estimating you just know what it will take.