Furnace Size Calculator
Enter your home's area and pick a mild, moderate, or cold climate zone to get an estimated furnace output in BTU per hour. The result is rounded to the nearest 1,000 BTU.
What this calculator does
The Furnace Size Calculator estimates your result instantly from the values you enter, using standard construction & home formulas. Enter your home's area and pick a mild, moderate, or cold climate zone to get an estimated furnace output in BTU per hour. The result is rounded to the nearest 1,000 BTU. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.
Result
About Furnace Size Calculator
Furnace heating output is often sized using a rough BTU-per-square-foot rate that rises with how cold the climate is. This calculator multiplies your home's area by the rate for the climate zone you choose.
The zones are mild at about 35 BTU/ft², moderate at about 45 BTU/ft², and cold at about 55 BTU/ft². The product is rounded to the nearest 1,000 BTU for a clean output figure.
Formula & Mathematical Calculation
Furnace Output (BTU/hr) = f(Home Area (ft²), Climate Zone)
Calculated according to standard construction formulas implemented in the calculator core.
Calculation Example
For an 1800 ft² home in a moderate zone (45 BTU/ft²):
- Raw output = 1800 × 45 = 81,000 BTU/hr
- Rounded to the nearest 1,000 = 81,000 BTU/hr
The tool shows Furnace Output 81,000 BTU and notes this is the rough output needed for that climate zone.
Key Benefits
- Applies a climate-based BTU-per-square-foot rate rather than a single flat number
- Offers mild, moderate, and cold zones to match your location
- Rounds the estimate to the nearest 1,000 BTU for an easy target output
- Free and runs in the browser with no sign-up
How to Use this Calculator
-
Enter the home area
Type the area to be heated in square feet into the Home Area (ft2) field, such as 1800. -
Choose the climate zone
Select Mild (~35 BTU/ft2), Moderate (~45 BTU/ft2), or Cold (~55 BTU/ft2) from the Climate Zone dropdown. -
Calculate
Press Calculate to see the estimated furnace output in BTU per hour.
Assumptions & Limitations
What the calculation assumes
- Every value you enter is treated as an exact, known amount.
- Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
- Results use standard published formulas and account only for the values entered above.
- Rounding is applied for display; the underlying calculation uses your full precision.
- Results are returned in the same units you enter, unless the tool states the output unit.
What it does not cover
- Furnace Size Calculator produces a calculated result for reference and learning, not a certified or official measurement.
- Real-world outcomes differ when tolerances, rounding rules or local standards apply.
- Definitions and standards vary by region and change over time; verify the convention that applies to your case.
- Figures are only as accurate as the inputs you supply — check them before acting on the result.
- For work where the result carries real consequences, confirm it against an authoritative source.
Frequently Asked Questions
Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.