How much paint you need depends upon the type and condition of the surfaces you'll be covering, the method of application, and the paint itself. Conditions vary considerably, so your best bet is to read the manufacturer's coverage figures, then expect to get slightly less.
To compute surface area, measure from the foundation to the eaves and multiply by the distance around the house. (Refer to Illustration 1 for help visualizing where you need to measure.) For each gable end, measure the distance from eaves to the peak, measure the width of the wall, and multiply the two. Then, divide the result by two.
If your home has narrow lap siding, add another 10 percent to your estimate. For textured materials, such as shingles or shakes, add 20 percent. Masonry and stucco -- both porous surfaces that soak up lots of paint -- can take up to 50 percent more.
If you buy premixed paint, you can always get more, and most stores will let you return unopened cans. Custom colors can be hard to match. So, buy an extra gallon; you can use the overage for various touch-up projects.