When taking the dimensions of each room or section, be sure to include interior and exterior walls. It is not enough to use clear space dimensions for calculating. Excluding the width of the walls will result in a miscalculation of the overall square footage. However, be sure that you don't count an interior wall twice. To avoid doing this, take your measurements from the outside face of a wall to the inside face of the opposite wall. When a section extends from exterior to exterior, measurements are to be taken from the outside face of the first wall to the outside face of the opposite wall.
Now that you have the home plan divided into manageable sections, you can begin your calculations. Take the width of a section and multiply it by the depth. This gives you the sq ft of that section. For example, a 18'x30' section (18 feet width, 30 feet depth) would have a total area of 540 sq ft. Now repeat this process for every section of the house that you have created. Remember to keep the conditioned and unconditioned calculations separate.
Tally the Sections
To find the total calculated square footage of conditioned home space, you need to add the square footage of each section that will be regulated by the HVAC unit. If you have six (6) sections on the house, each having 500 sq ft of space, the total area would be 3,000 sq ft. The unconditioned areas should be separated and listed by individual rooms. The following illustrates how the total square footage would be listed on a set of house plan blueprints.