Furnace size calculator
Enter the heated floor area, the climate and the efficiency of the furnace. The calculator gives the heat the house needs and the input rating printed on a furnace that delivers it. It uses a manufacturer’s rule of thumb, so treat the result as a range to check a quote against.
- Heat output needed80,000 to 100,000 BTU/h
- Input rating at 96% AFUE83,000 to 104,000 BTU/h
Heat output: 2,000 square feet × 40 to 50 BTU/h per square foot = 80,000 to 100,000 BTU/h.
A furnace is sold by its input rating. At 96% efficiency, an input of 83,000 to 104,000 BTU/h delivers that output. Furnaces come in steps of about 20,000 BTU/h, so choose the size nearest the range rather than the next one up.
A well-insulated, well-sealed house needs less than this rule of thumb gives. A Manual J load calculation is the way to find out how much less.
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Furnace size by square footage
Sizing starts with output, the heat a furnace delivers to the house. This table gives the output range for each climate, from a furnace maker’s rule of thumb.
| Floor area (sq ft) | Warm climate | Moderate climate | Cold climate |
|---|---|---|---|
| 1,000 | 30,000 to 35,000 | 40,000 to 50,000 | 50,000 to 60,000 |
| 1,500 | 45,000 to 52,500 | 60,000 to 75,000 | 75,000 to 90,000 |
| 2,000 | 60,000 to 70,000 | 80,000 to 100,000 | 100,000 to 120,000 |
| 2,500 | 75,000 to 87,500 | 100,000 to 125,000 | 125,000 to 150,000 |
| 3,000 | 90,000 to 105,000 | 120,000 to 150,000 | 150,000 to 180,000 |
Output in BTU/h. Warm is the South and Southwest, moderate is the Midwest and Mid-Atlantic, and cold is the Northeast and northern Midwest.
Input rating to look for
A furnace’s output equals its input rating times its AFUE, the efficiency rating. This table converts the moderate-climate output into an input rating at two efficiencies.
| Floor area (sq ft) | Output needed (BTU/h) | Input at 80% AFUE | Input at 96% AFUE |
|---|---|---|---|
| 1,000 | 40,000 to 50,000 | 50,000 to 63,000 | 42,000 to 52,000 |
| 1,500 | 60,000 to 75,000 | 75,000 to 94,000 | 63,000 to 78,000 |
| 2,000 | 80,000 to 100,000 | 100,000 to 125,000 | 83,000 to 104,000 |
| 2,500 | 100,000 to 125,000 | 125,000 to 156,000 | 104,000 to 130,000 |
| 3,000 | 120,000 to 150,000 | 150,000 to 188,000 | 125,000 to 156,000 |
Input ratings are in BTU/h, rounded to the nearest 1,000. A furnace with a 100,000 BTU/h input delivers 80,000 BTU/h at 80% AFUE and 96,000 BTU/h at 96%.
How the calculator works
output needed = floor area × BTU/h per square foot
input rating = output ÷ AFUE
The figures per square foot are 30 to 35 BTU/h for warm climates, 40 to 50 for moderate and 50 to 60 for cold. They come from Bryant, a furnace maker. They are a rule of thumb, not a standard. No government agency or standards body publishes BTU per square foot by climate for a whole house. Bryant tells readers to get a load calculation from a dealer.
Federal rules set a 78% minimum AFUE for furnaces made from January 1, 1992. They raised it to 80% for non-weatherized gas furnaces on November 19, 2015. These are government figures from 10 CFR 430.32.
ACCA Manual S is the published standard for equipment selection. An older edition, as summarized in an ICC journal article, put furnace capacity at 100% to 140% of the heating load. A 60,000 BTU/h load allows 60,000 to 84,000 BTU/h of capacity. This page could not confirm the limit for the current edition.
Limits of this calculator
- The result is a range, not a load. The load is the heat your specific house needs. Floor area does not show how well your house holds heat. A Manual J load calculation does.
- The climates are broad. Bryant’s highest figure is twice its lowest.
- Oversizing has a limit. The older Manual S edition capped furnace capacity at 140% of the load.
Sources
- Bryant, furnace sizing page: https://www.bryant.com/en/us/products/gas-furnaces/what-size-furnace-do-i-need/
- US Department of Energy, efficiency standards in 10 CFR 430.32 (eCFR): https://www.ecfr.gov/current/title-10/section-430.32
- ACCA, Manual S, equipment selection: https://www.acca.org/standards/technical-manuals/manual-s
- ICC Building Safety Journal, January and February 2009, summary of the Manual S limits: https://media.iccsafe.org/news/eNews/2009v6n8/hvac.pdf
Related
For one room, or for cooling, use the BTU calculator. To find the airflow a furnace needs, use the CFM calculator. Read how long a furnace lasts on the furnace lifespan page, or date one with the furnace age lookup. To read the input of the furnace you have now, use the model number decoder. For the per-square-foot figures and their limits, read how many BTU per square foot.
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Calculator last reviewed on October 5, 2026. Method and sources.