Determining the Square Footage
The square footage of your home is not the same as the square footage you will be painting, since the square footage of your home is calculated by measuring the area of the floors inside your house. To determine the square footage of the surfaces to be painted, measure the length and height of each surface in feet, then multiply the length by the height. Subtract 20 square feet for each door frame, and 15 square feet for each window.
Painting the House Yourself
Add up the cost of all the supplies you will need such as paint, rollers and brushes. One gallon of paint covers approximately 350 to 400 square feet. The cost of the paint can vary a lot depending on the quality of the paint you buy and whether it is interior or exterior paint. Good quality exterior paint starts at around $25 dollars per gallon as of February 2011, which means it would cost approximately 7 cents per square foot just for one coat of paint ($25 / 350 square feet). Good quality interior paint starts at roughly $12 a gallon, but can go as high as $50 per gallon (3.5 cents to 14 cents per square foot).
Hiring a Professional
You can save money by painting your house yourself or hiring a couple of your friends or neighbors to help you, but sometimes that is not realistic. If you have a large home or a house that is more than one story with lots of peaks and ledges, you will most likely want to hire a professional painter. When getting estimates from professional painters, they should provide you with a breakdown of their price by cost of supplies and labor.
Calculating the Total Cost
To calculate the cost per square foot of painting your house, divide the total cost of supplies or hiring a professional (depending on the route you take) by the total number of square feet to be painted. The number you are left with is the cost per square foot.