Just follow these simple steps.
Step 1 On a separate sheet of paper, make sketches of the home showing all sides. Measure the height of each side at the corner and multiply that by the width to attain the surface area of that side.
Step 2 Measure gables, dormers and any other area not included by the sides. For each triangular-shaped area measure from the base to the top, then multiply by 1/2 the base length. Then add all areas together.
Step 3 Measure the windows, doors, or any other area that will not be covered with siding. Multiply width x height for each area, then add all areas together.
Step 4 Take the total area from Steps 1 and 2 and add them together. Then, subtract total of Step 3 from the total of Steps 1 and 2. Always add 10% for waste and/or miscalculation of material.
Step 5 To determine the amount of siding squared needed, remember that 100 square feet of surface area equals one square, so divide your footage from Step 4 by 100.