This is always a tough one to answer because even if a contractor performs a 'Manual J' heat load, they can make compensations within the program to obtain the end result they are seeking. Keep in mind there are only 4 - 5 different sized furnaces each manufacturer produces and you want to install the size closest to your needs without being too small! As a close rule of thumb, you can multiply the square footage of your home which is above ground by the ceiling height to obtain your total cubic feet of airspace. Multiply that number by (4) if your home is 20 years or newer. Multiply by (4.5) if your home is 20 - 30 years old and multiply by (5) if it is over 30 years old. This answer will be the minimum Btu Output rating your home needs, not to be confused with the Btu Input rating.