One gallon of any BEHR paint is enough to cover between 250 to 400 square feet of surface area with one coat.
To determine the number of coats your project requires, first examine the surface you'll be painting. A porous, rough, textured or previously unpainted surface will generally require more paint, as will a project that involves a dramatic color change.
Estimate the size of the house body by measuring the perimeter of the house. Then, measure the height of the house from the foundation to the roofline, excluding gables. Multiply the perimeter of the home by the height.
If you have gables, multiply the width of the gable by its height, and divide by two. Do this for each gable and add the total to the body measurement.
To estimate the amount of paint you'll need for the trim, estimate the length of total trim. The average trim width is six inches. Multiply the total trim length by 0.5 feet. To estimate the gutter, determine the total length of the gutter. One linear foot of gutter approximates one square foot.
Once you add the trim and gutter calculations together, you'll know the additional paint you'll need for them. For doors, multiply the width by the height to determine the number of square feet.