Estimating the paint you'll need for a job is easy. Take a few minutes at home to measure the area to be painted. A gallon of paint will typically cover 450 square feet according to the manufacturer's calculations. It's safer to figure 400 square feet of coverage per gallon of paint. If you're buying 2 or more gallons of the same color, it's a good idea to mix them all together at home so color variations don't show up in the middle of a wall.
To determine the amount of paint required to cover a wall, multiply the height of the wall by its length, then divide by 400. This means a gallon of paint will cover a 10 X 15-foot room (two 10-foot walls and two 15-foot walls, 8 feet high) with one coat. Two coats will take 2 gallons.
However, there are other factors you should consider when calculating coverage.