How furnace sizing works
Furnace needs are driven by winter heat loss: the rule of thumb is 30–60 BTU per square foot, low in the South, high in the far North. This calculator multiplies your square footage by a climate-zone factor, corrects for insulation and ceiling height, and then converts the required output into the furnace input rating you will actually see on a spec sheet — because an 80% AFUE furnace only delivers 80% of its input BTU as heat.
Example: a home that needs 68,000 BTU of heat output requires an ~85,000 BTU-input furnace at 80% AFUE, but only ~71,000 BTU input at 96% AFUE. This is why high-efficiency furnaces can be a size smaller.
Furnace size chart (output needed)
| Home size | Warm (Z1–3) | Mixed (Z4) | Cold (Z5) | Very cold (Z6–7) |
|---|---|---|---|---|
| 1,200 sq ft | 36–42k | 54k | 62k | 72k |
| 1,600 sq ft | 48–56k | 72k | 83k | 96k |
| 2,000 sq ft | 60–70k | 90k | 104k | 120k |
| 2,500 sq ft | 75–88k | 113k | 130k | 150k |
Oversized furnaces: the most common US sizing mistake
Studies of installed furnaces repeatedly find them oversized by 40–100%. An oversized furnace heats the thermostat quickly and shuts off — short cycling — which causes big temperature swings between rooms, more wear on the heat exchanger and igniter, and lower real-world efficiency. If your current furnace runs only a few minutes at a time even in cold weather, replace it with a smaller, correctly sized unit, not the same size.
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
In a mixed climate (Zone 4), about 90,000 BTU of output — a 100,000 BTU-input furnace at 96% AFUE. In warm southern states 60–70k output is enough; in very cold climates plan on ~120k output.
Is it better to oversize a furnace?
No. Oversized furnaces short-cycle: uneven temperatures, more wear, higher bills. If you are between sizes and the house has decent insulation, take the smaller one — it will run longer, steadier and more comfortably.
What does AFUE mean?
Annual Fuel Utilization Efficiency — the share of fuel converted to usable heat. An 80% AFUE furnace sends 20% of the fuel's energy up the flue; a 96% condensing furnace only 4%. Output BTU = input BTU × AFUE.
How many BTU per square foot for heating?
From ~30 BTU/sq ft in the warm South to ~60 BTU/sq ft in the coldest northern zones, assuming average insulation. Good insulation can cut these numbers by 15% or more.
Should the furnace match my AC tonnage?
They are sized independently (heating vs cooling load), but they share the blower and ductwork. Your installer will pick a furnace whose blower supports the AC's required airflow — about 400 CFM per ton.