Furnace size calculator
Enter your home’s heated area and pick your climate. You’ll get the heat your home needs and the furnace input rating to shop for.
Heat your home needs
95,000 BTU per hour
- Furnace input rating
- 100,000 BTU per hour
- Nearest standard size
- 100,000 BTU
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Quick answers and tables: What size furnace do I need?
How it works
A common rule of thumb sizes heating by floor area: roughly 30 to 35 BTU per square foot in the warmest US climate zones, rising to 50 to 60 in the coldest. The calculator uses the middle of the range for your zone, then adjusts for ceilings taller or shorter than 8 feet and for insulation that’s better or worse than average.
That’s the heat your home needs. Furnaces are sold by their input rating, the fuel they burn, and an 80% efficient furnace only delivers 80% of it as heat. So the calculator divides by the efficiency (AFUE) and rounds up to the next common furnace size.
Example
With heated area 2,000 sq ft, climate: zone 5: cold (Ohio, Illinois, New England), average ceiling height 8 ft, insulation: average, furnace efficiency (AFUE): 95% (high efficiency), the answer is 95,000 BTU per hour.
Tips before you buy
- This is an estimate for budgeting and comparing quotes. A contractor should size the final unit with a Manual J load calculation, which accounts for your windows, walls and air leaks.
- Oversized furnaces cycle on and off often, which wastes fuel and wears them out faster. Bigger isn’t better.
- If you’re replacing an old furnace, don’t just match its size. Many older homes have oversized units, and added insulation or new windows reduce the load.
- Sealing air leaks and adding attic insulation can cut the size you need.
Common questions
What size furnace do I need for a 2,000 square foot house?
In a cold climate (zone 5) with average insulation, about 95,000 BTU of heat, which is a 100,000 BTU input furnace at 95% efficiency. In a warm climate (zone 3), about 75,000 BTU of heat, or an 80,000 BTU furnace.
What is AFUE?
Annual fuel utilization efficiency: the share of the fuel a furnace burns that becomes heat in your home. A 95% AFUE furnace turns 95% of its fuel into heat.
How many BTU per square foot do I need to heat a house?
Roughly 30 to 35 in the hottest US climates, 40 to 50 in mixed and cold climates, and 50 to 60 in the coldest, for a house with average insulation and 8-foot ceilings.