HVAC
Furnace Size Calculator
Estimate heating input and output for a space, with the distinction between the two made explicit.
Result
This is a preliminary sizing estimate using industry rules of thumb. Actual equipment selection requires a room-by-room load calculation (ACCA Manual J) and duct design (Manual D) by a qualified professional, oversized equipment short-cycles, dehumidifies poorly and fails early. Furnaces are rated by input; the heat actually delivered is input × AFUE.
Get a Free Business AuditWhat moves the number
- Design temperature for the location, which sets the worst case the furnace has to cover.
- Efficiency, since input BTU and output BTU differ by the AFUE rating.
- Envelope condition, which changes the load far more than floor area.
Where this one goes wrong
Confusing input and output rating. A furnace rated at a given input delivers less than that as heat, and sizing from the input figure produces a unit that is smaller than intended.
Before you quote
- Whether the gas line and meter can supply the input required.
- Whether the venting is correct for the efficiency category of the appliance.
- Whether the blower can move the airflow the heat exchanger needs.
Questions
About this calculator
What is the difference between input and output BTU?
Input is the fuel energy the furnace consumes. Output is the heat it actually delivers, which is input multiplied by the efficiency. A furnace and its output rating are different numbers and sizing must use the output.
Is an oversized furnace as bad as an oversized air conditioner?
It causes different problems rather than none: short cycling, temperature swings, uneven distribution and more wear. It is more forgiving than oversized cooling but it is not free.
Does a high efficiency furnace change the venting?
Yes. Condensing appliances produce acidic condensate and cooler flue gas, which requires different venting material and a condensate drain. A like-for-like swap is not always like for like.
The formula
input BTU = (area × climate factor) ÷ AFUE
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How to use this calculator
- Conditioned floor area (sq ft) (sq ft). Enter your own figure; the page starts at 1,800 sq ft.
- Climate zone (heating BTU per sq ft). Enter your own figure; the page starts at Cool north.
- Furnace efficiency (AFUE) (%). Enter your own figure; the page starts at 95%.
- Insulation quality. Enter your own figure; the page starts at Average.
The result updates as you type, so you can try a figure, see what it does to furnace input capacity, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Conditioned floor area (sq ft) | 1,800 sq ft |
| Climate zone (heating BTU per sq ft) | Cool north |
| Furnace efficiency (AFUE) | 95% |
| Insulation quality | Average |
What comes out
| Result | Value |
|---|---|
| Furnace input capacity main result | 85,263 |
| Heat output needed | 81,000 |
| Nearest standard size | 100,000 BTU input |
| Output per square foot | 45.0 |
You need about 81,000 BTU/h of heat output. At 95% AFUE that is 85,263 BTU/h input, nearest standard size 100,000 BTU input.
How sensitive the answer is
Holding everything else steady and changing only conditioned floor area (sq ft), this is what happens to furnace input capacity. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Conditioned floor area (sq ft) (sq ft) | Furnace input capacity |
|---|---|
| 900 sq ft | 42,632 |
| 1,350 sq ft | 63,947 |
| 1,800 sq ft example above | 85,263 |
| 2,700 sq ft | 127,895 |
| 3,600 sq ft | 170,526 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Conditioned floor area (sq ft) (sq ft)
- Measured in sq ft, starting at 1,800 sq ft.
- Climate zone (heating BTU per sq ft)
- Choose from Hot / humid south, Warm south, Moderate / mid, Cool north, Cold north, Very cold.
- Furnace efficiency (AFUE) (%)
- A percentage, between 0 and 100, starting at 95%.
- Insulation quality
- Choose from Older / poorly insulated, Average, Well insulated / new.
What the results tell you
| Result | What it is |
|---|---|
| Furnace input capacity | The headline result. A number. |
| Heat output needed | A number. |
| Nearest standard size | A category or planning selection, rather than a measured quantity. |
| Output per square foot | A decimal figure. |
Before you rely on this number
Sizing, capacity and load figures here are planning estimates. A real installation depends on the manufacturer's data for the specific equipment, the duct system it is going into, and a proper load calculation for the building. Use this to sanity-check a number or price a job, not to select equipment.
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