A common rule of thumb from HVAC maker Bryant puts a 2,000 sq ft home in a moderate climate at 80,000 to 100,000 BTU/h of heat. At 95% AFUE, that's a furnace input of about 84,200 to 105,300 BTU/h.
In this guide
- How to use the furnace size calculator
- The furnace size formula
- Furnace size chart by square footage
- Input vs. output: reading a furnace spec sheet
- Why a Manual J load calculation should set the size
- Oversized vs. undersized furnaces
- Furnace efficiency: AFUE and the input you need
- Gas furnace safety and carbon monoxide
- Tips and common mistakes
- Frequently asked questions
By hand, multiply the heated floor area by Bryant's BTU per square foot for your climate, then divide by the AFUE to get the input rating.
Treat that as a starting point, not the size to buy. A Manual J load calculation, which ACCA says national building codes require, is what a licensed HVAC contractor should size your furnace from. Have one? Enter its heat loss above, and our furnace size calculator gives the output and input to look for.
How to use the furnace size calculator
- Enter the heated floor area in square feet: the finished rooms the furnace heats. The square footage calculator adds up rooms for you.
- Pick your climate. These are Bryant's three regions: warm (South, Southwest), moderate (Midwest, Mid-Atlantic), and cold (Northeast, northern Midwest). Near a boundary, run both and compare.
- Enter the AFUE from the furnace's spec sheet. The default, 95%, is the lowest AFUE ENERGY STAR certifies for a gas furnace (in the U.S. South). Try 80 for a standard model.
- Add a Manual J heat loss if you have one, in BTU/h, from your contractor's load calculation. Leave 0 to use the rule of thumb.
The furnace size formula
- Heat needed (furnace output) = heated area (sq ft) × BTU per sq ft for your climate
- Furnace input = output × 100 ÷ AFUE
- With a Manual J heat loss: output to look for = heat loss to 1.4 × heat loss; input = output × 100 ÷ AFUE
- Results are rounded to the nearest 100 BTU/h
Bryant's guide says "a simple way to estimate furnace size is to multiply your home's square footage by 30 to 60 BTUs per square foot," and that "homes in colder climates require higher BTU output." The calculator reads those figures as output, the heat delivered to the house. AFUE links output to input: "a furnace with 80% AFUE delivers 80% of its input BTUs as heat," in Bryant's words. The Department of Energy (DOE) adds that at 90% AFUE, the other 10% escapes up the chimney and elsewhere.
Worked example (the calculator's default values)
A 2,000 sq ft home in a moderate climate, a 95% AFUE furnace, and no Manual J figure:
- Heat needed: 2,000 × 40 = 80,000 BTU/h at the low end, and 2,000 × 50 = 100,000 BTU/h at the high end.
- Input: 80,000 × 100 ÷ 95 = 84,210.5, rounded to 84,200 BTU/h; 100,000 × 100 ÷ 95 = 105,263.2, rounded to 105,300 BTU/h.
The calculator shows: "Estimated heat needed: 80,000 to 100,000 BTU/h of furnace output, or about 84,200 to 105,300 BTU/h input at 95% AFUE." and "Rule of thumb for a moderate climate (Midwest, Mid-Atlantic): 2,000 sq ft x 40 to 50 BTU per sq ft. Input = output x 100 / 95. Furnaces come in size steps: compare these figures with the output and input on the spec sheet."
With a Manual J heat loss
Say a contractor's Manual J puts the same house at a 60,000 BTU/h heat loss (an example figure). ENERGY STAR's rules for new homes allow a heating-only furnace an output of 100% to 140% of the heat loss, so the calculator shows: "For a 60,000 BTU/h heat loss, look for a furnace output of about 60,000 to 84,000 BTU/h, or 63,200 to 88,400 BTU/h input at 95% AFUE."
Furnace size chart by square footage
Heat needed (furnace output) in BTU/h, using Bryant's BTU per square foot. Bryant's own chart covers 1,000 to 2,500 sq ft; the other rows use the same figures. Divide by the AFUE for the input.
| Heated area | Warm (30 to 35 per sq ft) | Moderate (40 to 50) | Cold (50 to 60) |
|---|---|---|---|
| 1,000 sq ft | 30,000 to 35,000 | 40,000 to 50,000 | 50,000 to 60,000 |
| 1,200 sq ft | 36,000 to 42,000 | 48,000 to 60,000 | 60,000 to 72,000 |
| 1,500 sq ft | 45,000 to 52,500 | 60,000 to 75,000 | 75,000 to 90,000 |
| 1,800 sq ft | 54,000 to 63,000 | 72,000 to 90,000 | 90,000 to 108,000 |
| 2,000 sq ft | 60,000 to 70,000 | 80,000 to 100,000 | 100,000 to 120,000 |
| 2,500 sq ft | 75,000 to 87,500 | 100,000 to 125,000 | 125,000 to 150,000 |
| 3,000 sq ft | 90,000 to 105,000 | 120,000 to 150,000 | 150,000 to 180,000 |
Rules of thumb don't agree. Carrier's reference table lists 50,000 to 60,000 BTU/h as a "common range for many medium-sized homes" of 1,000 to 2,000 sq ft, well below Bryant's figures, and calls its ranges "not equipment-sizing recommendations." Carrier also says square footage alone should not be used to determine the correct furnace size.
Input vs. output: reading a furnace spec sheet
A furnace has two BTU ratings. Input is the fuel it burns per hour; output (heating capacity) is the heat that reaches your ducts. Spec sheets list both. Two Goodman model lines show how the AFUE links them:
| Input (BTU/h) | Output, 80% AFUE line (GM9S80) | Output, 96% AFUE line (GM9S96) |
|---|---|---|
| 40,000 | 32,000 | 38,440 |
| 60,000 | 48,000 | 57,660 |
| 80,000 | 64,000 | 76,880 |
| 100,000 | 80,000 | 96,100 |
| 120,000 | 96,000 | 115,320 |
Both lines come in steps of 20,000 BTU/h of input, so you pick the step whose output fits your number. For the default example (80,000 to 100,000 BTU/h of output), the 96% line's 100,000 input model, with 96,100 of output, lands inside the range; its 80,000 model, at 76,880, falls just below. Always compare output with output: two furnaces with the same 80,000 BTU/h input deliver 64,000 and 76,880 BTU/h.
Why a Manual J load calculation should set the size
ACCA calls Manual J "the national ANSI-recognized standard for producing HVAC equipment sizing loads" for homes, and DOE's heating and cooling guide says: "Insist that your contractor use the Air Conditioning Contractors of America (ACCA) Manual J to calculate the proper size of heating and cooling equipment."
A load calculation looks at what a square-foot rule can't. Carrier lists "the number and efficiency of windows, insulation levels, home orientation, ceiling heights, air leakage and the local climate." Bryant notes that a well-insulated, sealed home needs fewer BTUs and that ceilings above the standard 8 ft can need more; it gives no factor, so the calculator doesn't adjust for either. ENERGY STAR's HVAC design report for new homes uses room-by-room Manual J loads and a heating design temperature for the home's county.
That report also caps the furnace's output at 100% to 140% of the heat loss when it only heats. Paired with cooling, 100% to 140% is recommended and up to 400% is allowed.
DOE suggests improving the house first, with insulation or efficient windows, then having a heating contractor size the furnace. The attic insulation calculator shows how much ENERGY STAR recommends adding to an attic.
Oversized vs. undersized furnaces
Carrier says oversized furnaces heat too quickly and cycle on and off frequently (short cycling), which "can contribute to uneven temperatures, higher maintenance costs, reduced efficiency, and increased wear on system components." Undersized units "run continuously without adequately heating the home." Pick the step that fits the range; don't round up.
Furnace efficiency: AFUE and the input you need
A higher AFUE doesn't change the heat your home needs; it lowers the input that produces it. The input for the default example's 80,000 to 100,000 BTU/h of output:
| AFUE | Input for 80,000 BTU/h output | Input for 100,000 BTU/h output |
|---|---|---|
| 80% | 100,000 | 125,000 |
| 90% | 88,900 | 111,100 |
| 95% | 84,200 | 105,300 |
| 96% | 83,300 | 104,200 |
| 97% | 82,500 | 103,100 |
| 98.5% | 81,200 | 101,500 |
| AFUE | What it is | Source |
|---|---|---|
| 56% to 70% | Older, low-efficiency heating systems | DOE |
| 80% | What U.S. natural gas furnaces generally must deliver | Carrier |
| 90% to 98.5% | High-efficiency heating systems | DOE |
| 95% or more | ENERGY STAR gas furnaces, U.S. South (e.g., Texas, Florida, California) | ENERGY STAR |
| 97% or more | ENERGY STAR gas furnaces, U.S. North (e.g., Illinois, New York, Pennsylvania) | ENERGY STAR |
Gas furnace safety and carbon monoxide
DOE warns that a leaking furnace heat exchanger mixes combustion gases with house air, which can cause carbon monoxide poisoning, an important reason to have it inspected. It also suggests a sealed combustion furnace, which brings outside air directly to the burner and sends flue gases directly outside.
The Consumer Product Safety Commission (CPSC) recommends a professional inspection and service of the heating system every year, and carbon monoxide alarms meeting UL 2034 on every level of the home and outside sleeping areas. If an alarm sounds, move outside to fresh air immediately and call 911 or the fire department.
Replacing a gas furnace involves gas piping, venting, and wiring: hire a licensed HVAC contractor and get any permit your local code requires.
Tips and common mistakes
- Mixing up input and output. Compare the calculator's output range with the furnace's output (heating capacity).
- Copying the old furnace's size. It shows what was installed, not what the house needs now after new insulation or windows.
- Forgetting other heat sources. Carrier lists supplemental heat, such as fireplaces and wood-burning stoves, among the factors that affect furnace size. If you burn wood, the firewood calculator tells you how many cords are in your stack; for an electric space heater, the electricity cost calculator shows the kWh it uses.
- Sizing the air conditioner the same way. Cooling has its own load. For a single room with a window unit, the AC size calculator uses ENERGY STAR's room chart.
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
By Bryant's rule of thumb, about 60,000 to 70,000 BTU/h of heat output in a warm climate, 80,000 to 100,000 in a moderate one, and 100,000 to 120,000 in a cold one. Divide by the AFUE for the input, and let a Manual J set the final size.
How many square feet will an 80,000 BTU furnace heat?
Check whether 80,000 is the input or the output. Goodman lists 76,880 BTU/h of output for an 80,000 BTU/h input at 96% AFUE. By Bryant's rule of thumb, that matches roughly 2,200 to 2,560 sq ft in a warm climate, 1,540 to 1,920 in a moderate one, and 1,280 to 1,540 in a cold one.
Is it better to get a bigger furnace?
No. Carrier says an oversized furnace short cycles, which can mean uneven temperatures, higher maintenance costs, and more wear. ENERGY STAR's new-home rules cap a heating-only furnace at 140% of the heat loss.
What is the difference between furnace input and output BTU?
Input is the fuel the furnace burns per hour; output is the heat it delivers. Output is about input times the AFUE: Goodman lists 48,000 BTU/h of output for a 60,000 BTU/h input at 80% AFUE, and 57,660 at 96%.
Do I need a Manual J calculation to replace a furnace?
ACCA says a proper Manual J load calculation is required by national building codes and most state and local jurisdictions, and the Department of Energy tells homeowners to insist their contractor use it. Ask your building department what it requires.
What AFUE should a new gas furnace have?
Carrier says U.S. natural gas furnaces generally must deliver at least 80% AFUE. ENERGY STAR certifies gas furnaces at 95% or more in the U.S. South and 97% or more in the U.S. North.
Sources
- Bryant: What Size Furnace Do I Need? A Homeowner's Guide to BTU Sizing
- Carrier: Calculating Furnace Size
- ACCA: Manual J Residential Load Calculation
- U.S. Department of Energy: Guide to Home Heating and Cooling (PDF)
- U.S. Department of Energy, Energy Saver: Furnaces and Boilers
- ENERGY STAR: Furnaces Key Product Criteria
- ENERGY STAR: Single-Family New Homes National HVAC Design Report, Rev. 13 (2024, PDF)
- Goodman: GM9S80/GC9S80 Gas Furnace Specification Sheet (PDF)
- Goodman: GM9S96/GC9S96 Gas Furnace Specification Sheet (PDF)
- U.S. Consumer Product Safety Commission: Carbon Monoxide Questions and Answers
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