Furnace Size Calculator
Estimate the heating output your home needs in BTU/h and kW from its floor area, climate zone, and insulation, and the input rating of a furnace that delivers it at a given AFUE.
- Choose US or Metric. US shows the result in BTU/h, and Metric in kW.
- Under Floor area, keep Enter area and enter the heated floor area of your home, such as 2,000 sq ft. Leave out the garage and an unheated basement. If you don't know it, choose Calculate from rooms and enter the length and width of each heated room. Use Quantity for identical rooms.
- Choose your Climate zone. The DOE map and county list linked under the field show the zone for your county.
- Choose how well your home is insulated and sealed. The Heating factor fills in with the rule of thumb for your zone and insulation. If a contractor or a heat loss calculation gave you a factor, enter it instead. The reset link brings back the rule of thumb.
- If your ceilings are higher than 8 ft, enter the Ceiling height.
- Choose the AFUE of the furnace (80%, 90%, 95%, 97%), or choose Custom AFUE and enter the rating from its EnergyGuide label.
The area, factor, and AFUE fields accept whole numbers and decimals, such
as 2,000 or 92.1. The room and ceiling fields
also accept fractions and mixed numbers, such as 12 1/2.
How furnace size is estimated
The calculator multiplies your heated floor area by a heating factor for your climate, then adjusts for tall ceilings:
Heating output (BTU/h) = floor area (sq ft) × heating factor (BTU/h per sq ft) × ceiling adjustment
The heating factors come from the furnace maker Bryant's guide What Size Furnace Do I Need? A Homeowner's Guide to BTU Sizing, which uses about 30 to 60 BTUs per square foot depending on climate, insulation, and air leakage. Homes in colder places or with poor insulation fall toward the high end of each range, and well-insulated homes toward the low end. The calculator uses the low end for Good, the middle for Average, and the high end for Poor:
| Insulation | BTU/h per sq ft | W/m² |
|---|---|---|
| Zones 1–3 | ||
| Poor | 35 | 110.4 |
| Average | 32.5 | 102.5 |
| Good | 30 | 94.64 |
| Zone 4 | ||
| Poor | 50 | 157.7 |
| Average | 45 | 142 |
| Good | 40 | 126.2 |
| Zones 5–6 | ||
| Poor | 60 | 189.3 |
| Average | 55 | 173.5 |
| Good | 50 | 157.7 |
One BTU/h per sq ft is about 3.155 W/m². The calculator matches Zones 1–3 to Bryant's warm region (South and Southwest), Zone 4 to its moderate region (Midwest and Mid-Atlantic), Zones 5–6 to its cold region (Northeast and northern Midwest).
Climate zones
The zones are the eight IECC climate zones, listed in the calculator with the Building America region names. The DOE's Guide to Determining Climate Regions by County lists the zone for every county and has a map. Bryant names its regions by geography, so matching them to zone numbers is the calculator's own estimate. Some counties move between zones in newer editions of the IECC map, so check the guide for yours. The factor column is the rule-of-thumb range in BTU/h per sq ft.
| Zone | Example cities | Factor |
|---|---|---|
| Zone 1 | Miami, Honolulu | 30–35 |
| Zone 2 | Houston, Phoenix | 30–35 |
| Zone 3 | Dallas, Atlanta, Las Vegas, Los Angeles, San Francisco | 30–35 |
| Zone 4 | Baltimore, Kansas City, Seattle | 40–50 |
| Zone 5 | Chicago, Denver, Boston | 50–60 |
| Zone 6 | Minneapolis | 50–60 |
| Zone 7 | Fargo, Duluth | No rule of thumb |
| Zone 8 | Fairbanks | No rule of thumb |
Zones 7 and 8 are colder than Bryant's cold region, and no published rule of thumb covers them. Using the cold-region factor there would undersize the furnace, so the calculator asks you for a factor from a heat loss calculation or your contractor.
Your own heating factor
If you have a heat loss calculation for your home, such as one from the Heat Loss Calculator, divide its total by the heated floor area and enter the result as the heating factor. For example, a 72,000 BTU/h heat loss over 1,800 sq ft is 40 BTU/h per sq ft. Improving insulation and air sealing lowers the heat you need; to work out how much insulation to buy, use the Insulation Calculator.
Ceiling height
The rule of thumb assumes 8 ft ceilings. For taller ceilings, the calculator scales the output by ceiling height ÷ 8 ft, so 10 ft ceilings add 25%. That treats the extra height like extra air and wall to heat, which leans toward a larger estimate. Ceilings of 8 ft or lower, including the common 2.4 m, aren't adjusted down.
AFUE and furnace input
AFUE (Annual Fuel Utilization Efficiency) is the share of the fuel's heat that ends up heating your home over a season. A furnace's input rating is the heat it burns, and its output is the heat it delivers:
Furnace input (BTU/h) = heating output ÷ (AFUE ÷ 100)
An 80% AFUE gas furnace is a common standard-efficiency model, and Carrier calls 90% or higher high efficiency. ENERGY STAR gas furnaces are rated 95% or higher in the U.S. South and 97% or higher in the North. For an oil furnace (ENERGY STAR sets 87% or higher) or an older furnace, choose Custom AFUE. AFUE is a seasonal average, so compare the output capacity on the furnace's spec sheet with the heating output, too.
Examples
A 2,000 sq ft home in zone 4
With average insulation, the factor is 45 BTU/h per sq ft, so the home needs 90,000 BTU/h (26.38 kW) of heating output. An 80% AFUE furnace that delivers it is rated about 112,500 BTU/h input. The rule-of-thumb range for zone 4 is 80,000 to 100,000 BTU/h.
Tall ceilings
The same home with 10 ft ceilings needs 10 ÷ 8 = 1.25 times as much: 112,500 BTU/h, or about 140,625 BTU/h input at 80% AFUE.
A well-insulated home in a warm zone
A 1,500 sq ft home in zone 2 with good insulation needs 1,500 × 30 = 45,000 BTU/h. A 95% AFUE furnace that delivers it is rated about 47,368 BTU/h input.
Adding up rooms
Rooms of 20 × 15 ft, three of 12 × 12 ft, and one of 10 × 8 ft add up to 812 sq ft. In zone 6 with average insulation, that's 812 × 55 = 44,660 BTU/h, or about 55,825 BTU/h input at 80% AFUE.
Metric
A 150 m² home in zone 5 with average insulation uses 173.5 W/m², so it needs about 26.03 kW (88,802 BTU/h), or about 32.53 kW input at 80% AFUE.
FAQ
About 90,000 BTU/h of heating output in zone 4 with average insulation, or 80,000 to 100,000 BTU/h depending on insulation. In zones 5 and 6 it's about 100,000 to 120,000 BTU/h, and in zones 1 to 3 about 60,000 to 70,000 BTU/h. At 80% AFUE, a 90,000 BTU/h output takes a furnace rated about 112,500 BTU/h input.
About 30 to 60 BTU/h per square foot, according to Bryant: 30 to 35 in warm climates, 40 to 50 in moderate ones, and 50 to 60 in cold ones. Better insulation puts you toward the low end.
Input is the heat the furnace burns, and output is the heat it delivers to your home. Output = input × AFUE, so a 100,000 BTU/h input furnace at 80% AFUE delivers about 80,000 BTU/h.
Yes. An oversized furnace heats in short cycles, turning on and off often, which wears it faster and leaves rooms uneven. Size it to the heat your home needs rather than adding extra.
Look up your county in the DOE's Guide to Determining Climate Regions by County, linked under the Climate zone field. It lists the IECC zone for every U.S. county.
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