Quick answer: As a rule of thumb, homes need about 30–35 BTU per sq ft in warm climates, 40–50 in moderate climates and 50–60 in cold climates. A 2,000 sq ft home in the Midwest needs roughly 80,000–100,000 BTU of heat output.
A furnace that is too small leaves rooms cold on the worst days. One that is too big short-cycles: it blasts heat, shuts off, then starts again, wasting fuel and wearing out parts. Here is how to get in the right range.
Furnace Size Chart
| Home size | Warm (30–35) | Moderate (40–50) | Cold (50–60) |
|---|---|---|---|
| 1,000 sq ft | 30,000–35,000 | 40,000–50,000 | 50,000–60,000 |
| 1,500 sq ft | 45,000–52,500 | 60,000–75,000 | 75,000–90,000 |
| 2,000 sq ft | 60,000–70,000 | 80,000–100,000 | 100,000–120,000 |
| 2,500 sq ft | 75,000–87,500 | 100,000–125,000 | 125,000–150,000 |
| 3,000 sq ft | 90,000–105,000 | 120,000–150,000 | 150,000–180,000 |
Figures are BTU per hour of heat output, based on the climate ranges published by furnace maker Bryant.
Input vs Output: Don’t Skip AFUE
Furnaces are sold by input BTU, the fuel they burn. Their output is input × AFUE (Annual Fuel Utilization Efficiency).
Required input = Heating load ÷ AFUE
If your home needs 90,000 BTU of heat, an 80% furnace needs 112,500 BTU input, while a 95% furnace needs only about 95,000 BTU. That is why a high-efficiency furnace is often a smaller nameplate size.
Skip the math: enter your square footage, climate and AFUE to get the heating load and nearest standard furnace size. Open the free Furnace Size Calculator →
Other Things That Change the Size
- Ceilings higher than 8 ft add volume to heat.
- Poor insulation, old windows and air leaks raise the load; see our insulation calculator.
- Large glass areas and homes on slabs or over crawlspaces lose more heat.
- A finished basement adds heated area.
Get a Manual J Before You Buy
Rules of thumb are a starting point. A Manual J load calculation measures your walls, windows, insulation and air leakage and usually produces a smaller, more accurate size than square-footage rules. Ask installers to provide one; it costs little compared with years of oversized operation.
Furnace or Heat Pump?
Modern cold-climate heat pumps can heat many homes efficiently and also provide air conditioning. Compare sizes with our heat pump size calculator, and check ductwork with the duct size calculator.
Worked Example for a Cold Climate
Take a 2,400 sq ft home in Minnesota. At 50–60 BTU per sq ft the heating load is about 120,000–144,000 BTU per hour. With a 96% AFUE furnace, the middle of that range (132,000 BTU) needs about 137,500 BTU of input, so a 140,000 BTU furnace would be the rule-of-thumb choice. A Manual J for a well-insulated, tight house could easily come in lower, which is why the calculation is worth doing.
Single-Stage, Two-Stage or Modulating?
- Single-stage: runs at full output every time. Lowest cost, but cycles more.
- Two-stage: runs on a low setting most of the time and switches to high on cold days. Quieter and more even heat.
- Modulating: adjusts in small steps to match the load. Most comfortable and efficient, but most expensive.
Two-stage and modulating furnaces are more forgiving if the size is slightly large, because they spend most of their time at low output.
Common Furnace Input Sizes
| Input rating | Output at 80% AFUE | Output at 96% AFUE |
|---|---|---|
| 40,000 BTU | 32,000 BTU | 38,400 BTU |
| 60,000 BTU | 48,000 BTU | 57,600 BTU |
| 80,000 BTU | 64,000 BTU | 76,800 BTU |
| 100,000 BTU | 80,000 BTU | 96,000 BTU |
| 120,000 BTU | 96,000 BTU | 115,200 BTU |
Signs Your Furnace Is the Wrong Size
- It turns on and off every few minutes (too big).
- It runs constantly on cold days and the house still feels chilly (too small).
- Some rooms are much colder than others, which may be a duct problem rather than size.
Sources
Frequently Asked Questions
What size furnace do I need for a 1,500 sq ft house?
About 45,000–52,500 BTU of output in warm climates, 60,000–75,000 in moderate climates and 75,000–90,000 in cold climates.
Is a 100,000 BTU furnace too big?
It depends on your home. For a well-insulated 2,000 sq ft home in a moderate climate, a 100,000 BTU input furnace at 95% AFUE is often about right.
Does AFUE affect furnace size?
Yes. Higher AFUE means more of the input becomes heat, so you need a smaller input rating for the same home.
What happens if a furnace is too big?
It short-cycles, heats unevenly, wastes fuel and wears out faster.
How many BTU do I need per square foot?
About 30–35 in warm climates, 40–50 in moderate climates and 50–60 in cold climates as a rule of thumb.
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Try the free Furnace Size Calculator or browse all home improvement calculators.