how much does one gallon of paint cover
For a relatively smooth surface, like old paint in good condition, or primed wood, it generally takes about 1 gallon per each 400 square feet. Thus, measure the space to be painted and calculate the area in square feet. Divide the area by 400 to get the number of gallons per coat. Rough surfaces like stucco will take much more paint, and one gallon may do only 200 square feet, so use 200 sq. ft./gallon for this type surface.