Most 1,200-square-foot homes needs a furnace with an effective output of 40,000 to 50,000 BTUs. The effective BTU output, which is based on its efficiency rating, is a percentage of the BTU input rating. For example, a 50,000-BTU furnace with an efficiency rating of 80 percent provides a 40,000-BTU output.

Factors such as construction type, building date, quality of insulation, quality of windows and doors, and the number and size of rooms are used to determine the exact size of furnace required for any home. The most effective method for choosing the right furnace is to obtain furnace estimates from local, licensed contractors. There are also free and paid software programs and websites that provide furnace size estimators.