All you need is a tape measure and some simple math
1. Below the Roof-line:
Measure the distance along each side of your home. Multiply that number by the height up to the roof-line.
2. Above the roof-line:
Measure how many feet the tallest point of the peak rises above the side height measured in step 1 and multiply that number by 1/2 the width of the wall. Add this figure to the total you calculate in step 1.
3. Windows and Doors:
Determine the area of all windows and doors using the same width x height calculation and subtract each from your total.
4. Gallons of paint:
Divide the total area by: 350 – 400, which is the number of square feet that one gallon will cover.
5. Trim painting:
Multiply the number of doors by 21, the number of windows by 15, and add up the results. Divide by 350 – 400 to get gallons of trim paint you will need.