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The quick rule of thumb (and why it is only a rule of thumb)
Furnaces are rated by BTU of heat input per hour. The fast estimate uses a BTU-per-square-foot figure that rises as the climate gets colder. A ton of cooling does not map cleanly to a furnace, because heating load depends far more on outdoor design temperature than cooling load does. Here is the starting table:
| Climate zone | BTU per sq ft | 2,000 sq ft example |
|---|---|---|
| Hot south | 30 to 35 | 60,000 to 70,000 BTU |
| Moderate / mid | 40 to 45 | 80,000 to 90,000 BTU |
| Cold north | 50 to 60 | 100,000 to 120,000 BTU |
That gets you in the right neighborhood. It does not get you to the right answer, because two homes of the same size can have very different heat loss. Once you have a working number, you can browse gas furnaces and narrow from there.
What actually changes your size
Climate and design temperature
A furnace is sized to the coldest design day for your area. The same house needs far more input in Minnesota than in Georgia.
Insulation, windows, and air sealing
A tight, well-insulated home holds heat. A drafty older home with single-pane glass loses it, and needs more input to keep up.
AFUE: input versus delivered heat
AFUE is the percentage of fuel the furnace turns into usable heat. A 96% AFUE furnace delivers 96,000 BTU from a 100,000 BTU input. An 80% AFUE furnace delivers 80,000. Higher AFUE means more delivered heat from the same input and lower fuel use over the season.
Match the furnace to the blower and ductwork
An oversized furnace short-cycles, overheats the duct, and wastes fuel. The furnace also has to move enough air through your existing ductwork. Pair the input rating with the right blower and confirm your duct system can carry the airflow before you size up.
Furnace BTU Calculator
Estimate the heating input your home needs, then jump to matching furnaces.
Estimate only. Confirm with a Manual J load calculation and check your ductwork capacity.
Shop high-efficiency furnace systems
Variable-speed, high-AFUE furnaces for a typical home. Current pricing shows on each product page.
Common furnace sizing questions
Is a bigger furnace better?
How many BTU do I need for 1,500 sq ft?
What is a good AFUE rating?
Does furnace size depend on my ductwork?
25 years sizing and shipping HVAC systems to homeowners and contractors.