The size of furnace needed for a home depends on the size of the home or the geographic location of the home. The needed BTUs are calculated based on the home size and the desired amount of heat.
British thermal units are the standard measurements used to determine the heating capacity for a furnace. The square footage of a home is used to determine how many BTUs a furnace needs to heat the home. A home located in a cold climate requires more BTUs to heat than a home in a warmer climate. Other issues that factor into the needed furnace size are the amount of insulation in the home, as well as the size and placement of windows.