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This furnace size calculator estimates what size furnace your home needs. Enter the heated area, your climate and the furnace efficiency (AFUE), and it gives the heating load range in BTU per hour, the matching furnace input rating and the nearest standard size.
How to Use the Furnace Size Calculator
- Enter the heated floor area in square feet.
- Choose the climate that matches your region.
- Enter the average ceiling height.
- Enter the AFUE of the furnace you are considering (80% for standard, 90–98% for condensing furnaces).
Furnace BTU by Climate
| Climate | BTU per sq ft | 2,000 sq ft home |
|---|---|---|
| Warm (South, Southwest) | 30–35 | 60,000–70,000 |
| Moderate (Midwest, Mid-Atlantic) | 40–50 | 80,000–100,000 |
| Cold (Northeast, northern Midwest) | 50–60 | 100,000–120,000 |
These are rule-of-thumb ranges from furnace maker Bryant. The calculator increases them in proportion to ceilings above 8 ft.
Input vs Output: Why AFUE Matters
Furnace input (BTU/h) = Heating load ÷ (AFUE ÷ 100)
Furnaces are labeled by input BTU. An 80% AFUE furnace with a 100,000 BTU input delivers 80,000 BTU of heat; a 96% furnace delivers 96,000 BTU. That is why a high-efficiency furnace can have a smaller input rating for the same house.
Example
A 2,000 sq ft home in a moderate climate needs about 80,000–100,000 BTU of heat. At 95% AFUE, the middle of that range (90,000 BTU) calls for about 95,000 BTU input, so a 100,000 BTU furnace.
Why Not Just Go Bigger?
- Oversized furnaces short-cycle, which wastes fuel and wears parts.
- They heat unevenly and can be noisier.
- A Manual J load calculation, which accounts for insulation, windows and air leakage, gives the most accurate size.
Considering a heat pump instead? Try the heat pump size calculator.
Sources
Frequently Asked Questions
What size furnace do I need for a 2,000 sq ft house?
About 60,000–70,000 BTU of heat in warm climates, 80,000–100,000 BTU in moderate climates and 100,000–120,000 BTU in cold climates.
How many BTU per square foot for heating?
Rules of thumb range from about 30 BTU per sq ft in warm climates to 60 BTU per sq ft in cold climates.
What does AFUE mean?
Annual Fuel Utilization Efficiency: the percentage of fuel energy that becomes heat in your home.
Is a bigger furnace better?
No. An oversized furnace cycles on and off too often, wasting energy and reducing comfort.
Read the guide: What Size Furnace Do I Need? BTU by Square Footage and Climate
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