The best drywall estimating services will also measure the work space before you begin. After determining the size of the work area, the drywall estimator will measure the square footage of each area. A simple rectangular space will be easy to calculate, so all you need to do is multiply the width by the height of the wall. The same goes for sloped walls. Roof areas will also be calculated the same way. So, the first step in getting a dependable evaluator for your drywall project is to find a service that does both.