There are a lot of decisions to make when painting your house or even just one room. You have to think about the color, trim, and calculate how much time it’s going to take. Before you get started, you’ll also want to figure out how much paint you need. You don’t want to get halfway through your paint job only to realize you don’t have enough paint to finish it.

First, measure the room from wall to wall and floor to ceiling. Multiply the numbers for each wall and add them together to calculate the square footage of the walls. Subtract the space taken up by windows and doors—20 feet for each door and 15 feet for each standard window.

One gallon of paint will cover about 350 square feet. Of course, you’ll need to take into consideration the wall material and whether you’ll want or need an extra coat. When in doubt, always round up and buy in bulk. If you have extra, you can always save it for touch