Calculate the square footage of your home by measuring the dimensions of each room, hallway, closet and other space in your house that you intend to heat or cool. Multiply the dimensions of each room together to find the room's area and sum up all the areas to find the total square footage of your home. You may already know this number from when you purchased the home.
Determine your climate zone by looking at the map provided in the reference section. Zones range from zone 5 in the coldest northern states to zone 1 in the warmest southern states. The zone helps estimate the average high and low temperatures in your area to know how much work the furnace or air conditioner will need to do to get the home to a comfortable temperature.
Look up the recommended size of air conditioner for your climate zone and square footage in the chart provided in the references section. For example, a home located in zone 3 that is about 1,700 square feet would need an air conditioner with a 3-ton capacity.
Multiply the number of tons by 12,000 to find how many BTUs, or British Thermal Units, capacity the air conditioner needs to have.
Assess whether your home's insulation is poor or good. Poorly insulated homes have older or larger windows and may get drafty during the winter. Well-insulated homes have newer windows, good insulation materials, and may be two stories, which helps provide insulation.
Multiply the square footage of your home by the appropriate number of BTUs per square foot for your climate zone to estimate your furnace size. Choose the smaller number in the range if your home is well-insulated and the larger number if your home is poorly insulated. Multiply by 30 to 35 for zone 1, 35 to 40 for zone 2, 40 to 45 for zone 3, 45 to 50 for zone 4 and 50 to 60 for zone 5.