Before you begin painting your home's interior walls, ceiling, woodwork, doors, or windows, you need to estimate the amount of paint you'll use. Estimates require specific calculations for each surface you want to paint.
To estimate the amount of paint you need in order to cover the walls of a room, add together the length of all the walls and then multiply the number by the height of the room, from floor to ceiling. The number you get is the room's square footage. Is that math class coming back to you now?
Now you have to determine how much of that square footage is paintable surface area. Because you use a different paint on the doors and windows, subtract those areas from the room total. No sweat, just subtract 20 square feet for each door and 15 square feet for each average-sized window in the room. You end up with a number that is close to the actual wall area you have to cover with paint.
In general, you can expect 1 gallon of Benjamin Moore paint to cover about 400-450 square feet. You need slightly more than a gallon if the walls are unpainted drywall, which absorbs more of the paint. You also need to consider whether to paint more than one coat. If you're painting walls that are unfinished, heavily patched, or dark in color, plan on applying two coats of paint.
When painting a dark color, pros often add a color tint to the white primer. Tints for both latex or alkyd paints are available at most paint stores. For best results, choose a tint shade that's closest to the top coat color.
Now for the clincher of the math problem. Divide the paintable wall area by 425 (the square-foot coverage in each gallon can) to find the number of gallons of paint you need for the walls. You can round uneven numbers; if the remainder is less than .5, order a couple of quarts of wall paint to go with the gallons; if the remainder is more than .5, order an extra gallon. Of course, buying in bulk is usually more economical, so you may discover that 3 quarts of paint cost as much as a gallon.