After you have done the multiplication, subtract about 40 sq feet per window, and about 60 sq feet per door. Divide by 400 (interior paint covers 400 sq feet per gallon). The number that remains, is how many gallons you will need. If doing multiple wall colors, then you should do this process for each room (or sets of rooms) with a particular color. 
Homeowners report that painting a home’s exterior costs an average of $2,923 with a typical range between $1,734 and $4,119. The average price per square foot ranges from $0.50 to $3.50 depending on your location, condition of your exterior and accessibility. Stucco and brick cost an average of $1 per square foot more than vinyl or wood. The average 2,500 square feet home costs an average of $4,000, though it can range from $1,250 to $8,750.