The size of the house, the condition of the surfaces, the type of coating you select, and the method of application are all factors that will determine the amount of paint you need to buy.
Narrow lap siding, shingles or shakes, masonry, or stucco exterior walls can take 10 to 50 percent more paint than smooth, flat walls.
Airless sprayers, which apply the equivalent of two coats of paint at one time, may require twice as much paint as brushes, rollers, or pads for the same surface dimensions.
You can get more standard, premixed paint if you run out. Or, if you buy too much, many stores will give credit or refunds for unopened gallons. With custom-colored paints, however, it may be difficult to get a precise match if you run short. Calculate your needs carefully, then buy an extra gallon for insurance.
To determine how much paint your house needs, measure the house's perimeter. Then multiply that figure by the height, excluding gable ends. Take the measurements with a steel tape measure, or reel out a ball of twine around the house and mark and measure the twine. If you will use a different paint on your home's trim, subtract 21 square feet for every door and 15 square feet for each typical window. Divide the final figure by the square-foot coverage specified on the can of paint to determine the number of gallons you will need for one coat.
If your house has gables, you can estimate by just adding 2 feet to the height when making your calculations. For more precision, measure the width of the gable wall and multiply that figure by its height. Divide the final figure by 2 to determine the gable's square-foot dimensions.
For trim paint, the rule of thumb is 1 gallon for every 6 gallons of wall paint. To be more accurate, you'll have to figure the areas of doors, windows, and shutters. For gutters, a linear foot is about equal to a square foot, so for 50 feet of gutter, buy sufficient paint to cover 50 square feet.