Free Furnace Size Calculator calculator
Size a gas furnace by home square footage and climate zone — get required BTU/hr and the recommended standard furnace size in seconds.
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BTU/hr estimates are based on simplified Manual J guidelines. Consult a licensed HVAC contractor for a full load calculation before purchasing.
Results are estimates. Consult a professional.
How the furnace size calculator works
The calculator multiplies your home's square footage by a heating factor (BTU per square foot) to get the required BTU/hr heating load. It then finds the nearest standard furnace size at or above that load — because undersizing leaves the home cold on the coldest days, and moderate oversizing is acceptable while extreme oversizing causes short-cycling problems.
Heating factors by climate zone
The heating factor captures how cold your climate gets — colder zones need more BTU per square foot to maintain comfortable indoor temperatures on design-day conditions. Using the right factor is the single most important variable in sizing a furnace correctly.
| Climate zone | Location examples | BTU/ft² |
|---|---|---|
| Zone 1–2 | Florida, South Texas, Hawaii | 30–35 |
| Zone 3–4 | Tennessee, Missouri, Oregon coast | 40–45 |
| Zone 5 | Ohio, Pennsylvania, Colorado | 45–50 |
| Zone 6 | Minnesota, Montana, New England | 50–60 |
| Zone 7 | Northern Canada, Alaska | 60–70 |
A worked example: 1,500 ft² home in a moderate climate
David has a 1,500 ft² two-story home in the Midwest (climate zone 3–4). He wants to replace his aging furnace and needs to know what size to buy.
Step 1 — Calculate required BTU/hr
1,500 × 45 = 67,500 BTU/hr of heating capacity required.
Step 2 — Find the nearest standard size
The next standard furnace above 67,500 BTU/hr is 80,000 BTU/hr (80k). The 60,000 BTU/hr model would be undersized by 10,500 BTU/hr.
Step 3 — Convert to kW
67,500 ÷ 3,412 ≈ 19.8 kW. This is the electrical equivalent of David's heating load.
Standard furnace sizes and what they cover
Gas furnaces are manufactured in standard BTU/hr increments. The right size is the smallest that meets or exceeds your calculated load — going to the next size up is fine, but jumping two or three sizes above the load causes the short-cycling problems described in the section below.
| Furnace size | Typical home size (moderate climate) | Typical home size (cold climate) |
|---|---|---|
| 40,000 BTU/hr | Up to 900 ft² | Up to 700 ft² |
| 60,000 BTU/hr | 900–1,300 ft² | 700–1,100 ft² |
| 80,000 BTU/hr | 1,300–1,800 ft² | 1,100–1,400 ft² |
| 100,000 BTU/hr | 1,800–2,200 ft² | 1,400–1,700 ft² |
| 120,000 BTU/hr | 2,200–2,700 ft² | 1,700–2,000 ft² |
| 150,000 BTU/hr | 2,700–3,300 ft² | 2,000–2,500 ft² |
| 180,000 BTU/hr | 3,300–4,000 ft² | 2,500–3,000 ft² |
| 200,000 BTU/hr | 4,000+ ft² | 3,000+ ft² |
Moderate climate = zone 3–4 (45 BTU/ft²). Cold climate = zone 5–6 (55 BTU/ft²).
These ranges are rule-of-thumb estimates for well-insulated homes. Older homes, homes with high ceilings, or homes with large window areas may need a factor or size step higher.
AFUE ratings and what they mean
AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel that becomes heat. A 95% AFUE furnace converts 95% of the gas to heat and loses only 5% up the flue. The federal minimum for new gas furnaces is 80% AFUE in most US regions.
- 80% AFUE — standard efficiency; meets federal minimum; lower upfront cost but higher operating cost.
- 90–92% AFUE — mid-efficiency; the sweet spot for most mild-to-moderate climates.
- 95–98% AFUE — high efficiency; condensing furnace with PVC flue; best for cold climates or high gas prices.
- 97–98% AFUE modulating — top tier; variable-speed blower and modulating gas valve; quietest and most efficient but highest price.
Why oversizing a furnace is a problem
A furnace that is too large for the home short-cycles — it reaches setpoint quickly, shuts off, and then fires again within minutes. This wastes fuel, stresses the heat exchanger, and creates uneven temperatures throughout the house.
- Short cycling shortens furnace life — frequent starts stress the heat exchanger and ignition system.
- Humidity is not controlled properly — a short run doesn't give the air handler enough time to dehumidify.
- Ductwork and registers make more noise — a sudden blast of hot air from an oversized unit causes banging and expansion noise.
- A furnace sized 20–25% above the load is fine; 50%+ oversizing causes all of the above problems.
How accurate is this furnace size calculator?
This is a simplified rule-of-thumb based on BTU per square foot — the same method used by HVAC rule-of-thumb guides and preliminary contractor quotes. It gives a reliable ballpark in seconds without needing detailed home data.
A proper ACCA Manual J calculation accounts for insulation R-values, window area and U-factors, infiltration, orientation, ceiling height, and internal gains. For a finished installation, always get a full Manual J from a licensed HVAC contractor before purchasing equipment.