Furnace Size Calculator
Estimate home heating output BTU, furnace input BTU by AFUE, airflow CFM, and the sizing factors that change load by climate, ceiling height, insulation, and windows.
🏠Home and zone presets
📏Furnace sizing inputs
Furnace sizing result
⚙Formula factors used
📊Furnace sizing reference tables
| Climate selection | Calculator value | Typical range | Best match |
|---|---|---|---|
| Mild | 25 BTU/ft² | 25 to 30 BTU/ft² | Short, mild heating season |
| Warm mixed | 30 BTU/ft² | 30 to 35 BTU/ft² | Light winter load with occasional cold snaps |
| Average | 40 BTU/ft² | 35 to 40 BTU/ft² | Balanced heating season and typical construction |
| Cool | 45 BTU/ft² | 40 to 45 BTU/ft² | Cool winter climate or exposed site |
| Cold | 50 BTU/ft² | 45 to 50 BTU/ft² | Snow climate or longer heating season |
| Very cold | 60 BTU/ft² | 55 to 60 BTU/ft² | Very low winter design temperatures |
| Factor | Low load | Normal load | High load |
|---|---|---|---|
| Ceiling height | 7.5 ft = x0.94 | 8 ft = x1.00 | 10 ft = x1.25 |
| Insulation | Excellent = x0.85 | Average = x1.00 | Poor = x1.28 |
| Windows | Sheltered = x0.92 | Typical = x1.00 | Glass-heavy = x1.30 |
| Area unit | 1 m² = 10.764 ft² | 1 ft² = 0.0929 m² | Use only heated area |
| Required output | 80% AFUE input | 90% AFUE input | 96% AFUE input |
|---|---|---|---|
| 40,000 BTU/hr | 50,000 BTU/hr | 44,444 BTU/hr | 41,667 BTU/hr |
| 60,000 BTU/hr | 75,000 BTU/hr | 66,667 BTU/hr | 62,500 BTU/hr |
| 80,000 BTU/hr | 100,000 BTU/hr | 88,889 BTU/hr | 83,333 BTU/hr |
| 100,000 BTU/hr | 125,000 BTU/hr | 111,111 BTU/hr | 104,167 BTU/hr |
| Output BTU/hr | 40 F rise | 50 F rise | 60 F rise |
|---|---|---|---|
| 40,000 | 926 CFM | 741 CFM | 617 CFM |
| 60,000 | 1,389 CFM | 1,111 CFM | 926 CFM |
| 80,000 | 1,852 CFM | 1,481 CFM | 1,235 CFM |
| 100,000 | 2,315 CFM | 1,852 CFM | 1,543 CFM |
💡Sizing tips
When it comes to sizing a furnace, there are some variables that has to be understood to size right, and you can’t just go by what is printed on the box. Just because a furnace is bigger doesn’t make it any more efficient.
An oversize furnace will cause the system to short-cycle. It blows out hot air for a short period of time and then turns off and starts over again. Your energy bill goes up and air becomes dry and stuffy. Because the system isn’t running long enough, it can’t dehumidify area well, causing the temperature to swing.
How to Choose the Right Furnace Size
Your first variable is the size of the space you want to heat (the heated floor area). Next, it factors in your climate zone; a house in Minnesota will have a greater heating load then an identical home in Florida. It includes ceiling height, too. More floor space mean more air volume to heat. A standard 8-foot ceiling is used as the default; double-height spaces and vaulted rooms greatly enlarge the volume. The calculator takes this into account; and prevents you from underestimating the load.
Another key ingredient is quality of insulation, and whether house is sealed up properely to hold the heat in. If you have an old house with no insulation in the attic, drafty windows, etc., it will lose heat rapidly once the furnace shuts down. To make up for this, you’ll need more output because all that heat are leaking out through those gaps.
You rated your house’s envelope truthfully in the tool. For example, if you have gaps in your siding or single-pane window, you need higher output. The point isn’t necessarily what kind of heat you’re producing from the furnace; it’s how much of that heat are staying put.
You may have seen efficiency ratings called AFUE, which can be confusing. A high-efficiency furnace with an AFUE of 96 percent will convert nearly all gas it burns into useful heat. Because the older 80 percent unit wastes some energy going up the flue, the calculator factors in your unit’s efficiency when determining how many BTUs is needed.
What you should know is that the input is what the utility bills you for; the output warms your house. And this matters to your wallet, because a highly efficient unit delivers same level of comfort using less fuel. That’s why the reference table show how much less fuel it takes for such a unit to provide the same comfort.
Heat doesn’t magically appear; air carries it. So airflow is part of the equation, too. A big variable is how much heat need to be distributed (CFM stands for cubic feet per minute). Another variable is what size of ducts is required to deliver that heat without making the noise too loud. Pushing more air into under-sized ducts will cause pressure issues, which can be solved with better ducts. Or sometimes better sealing, but not necessarily a larger furnace.
More isn’t always better, it typicaly just covers up a leaky building shell. The number in this calculator is a planning estimate and you can get a realistic range for your own situation. Go with those numbers to your contractor so he/she would of know what to discuss with you and don’t just go out and buy the lowest priced unit. Make sure the contractor confirms the load calculation. If they just guess or fail to provide any details other than square footage, ask them to reconsider.
The right size is good for your home’s comfort, your energy bill, and also the life of the equipment. You want one that operate in long, steady cycles to keep your home comfortable.
