Measuring walls and ceilings
For walls, measure the length of each wall along the floor and the height from floor to ceiling. Measure doors and windows separately so they can be subtracted. For ceilings, the floor below a flat ceiling has the same size, so measure the floor wall to wall. For sloped walls or ceilings, measure along the surface and enter the total area.
Coverage and coats
How far a gallon goes depends on the product, the surface and how you apply it, so every calculator here asks for the coverage printed on your can or the product’s technical data sheet. Multiply by the number of coats the label recommends, and treat primer as a separate product with its own coverage.