Question: how do you know how much paint to buy?
Answer: Subtract 20 square feet for each door (20 × 2 = 40) and 15 square feet for each window (15 × 2 = 30) to find the actual amount of wall area you're painting. 4. Divide this figure by the paint coverage (350 square feet per gallon), and the result is the number of gallons to purchase.