To estimate paint cost, determine the number of gallons you need by dividing the wall square footage by 350 and then multiplying this result by the desired number of coats. Round your answer up and multiply it by the cost of a gallon of paint. For example, if one gallon of paint costs $42 and you need 2.6 gallons, round 2.6 to 3 and multiply 3 x 42 for a total cost of $126.