1. Determine the average height of the house. For flat roof types, measure the distance from the foundation to the eaves. For pitched roofs, add two feet.
2. Measure the distance around the house - measure around the foundation.
3. Multiply the average height by the distance around the foundation. This total equals the surface area.
4. Divide the surface area by 500 (because most house paint covers 500 square feet per American gallon). Your result equals the number of gallons required for each coat.
5. For the trim - the average six to eight room house requires about 1 gallon of paint.