Here are some suggestions to help you estimate how much paint to buy for a given job:
 Compute the number of square feet of surface to be covered: First determine the distance around the house, in feet. Then find the average height to the eaves and add two feet to that figure. (This automatically takes care of the paint you'll need for the gables.) Finally, multiply the former result by the latter.
 Ascertain from your paint dealer approximately how many square feet each gallon of undercoat covers. Then divide the total area by this figure.
 Similarly, learn the coverage of a gallon of topcoat house paint and divide as in  .
Example: Let's say the house is 20 by 40 feet with an average height to the eaves of 13 feet. Multiplying 120 (the perimeter) by 15 (average height to eaves plus two feet), we find that the surface area is about 1,800 square feet. A good grade of undercoat or primer covers about 450 square feet per gallon, and a top-quality house paint covers 500 square feet per gallon. A little simple arithmetic reveals that four gallons of undercoat and three and a half of house paint are needed.
Before you place your order, however, there's another factor to be considered: the condition of the surface. If it's unusually rough, heavily textured, dry or porous, it may require 20% more paint for the first coat. Remember also that corrugated metal requires one-third more paint than usual because there's more area to cover per square foot. For the same reason, narrow-lap siding uses up to 10% more paint than wide.
No matter what kind of surface you're painting, allow 48 hours' drying time between coats.
For more information about house painting, check out these articles: How to Start a House Painting Business and House Painting Tips.