How to use the furnace size calculator
- Enter the heated floor area. Leave out garages and unheated basements.
- Pick your climate zone and how well the house is insulated and sealed.
- Enter the AFUE efficiency of the furnace you’re considering (80% for standard models, 90–98% for condensing ones).
The formula
- BTU per sq ft = the zone’s range: low end for a well-insulated house, middle for average, high end for a drafty one
- Heat needed = floor area × BTU per sq ft
- Furnace input = heat needed ÷ AFUE
- Size = input rounded up to the next 20,000 BTU (at least 40,000)
Worked example: 2,000 sq ft in zone 4, average house
- BTU per sq ft: halfway between 45 and 50 = 47.5
- Heat needed: 2,000 × 47.5 = 95,000 BTU/h
- Input at 95% AFUE: 95,000 ÷ 0.95 = 100,000 BTU
- Size: a 100,000 BTU furnace (120,000 at 80% AFUE)
Furnace size by home size and zone
Average insulation, 95% AFUE. Zones 6–8 use zone 5’s figures.
| Floor area | Zone 1 | Zone 3 | Zone 5 |
|---|---|---|---|
| 1,000 sq ft | 40,000 | 60,000 | 60,000 |
| 1,500 sq ft | 60,000 | 80,000 | 100,000 |
| 2,000 sq ft | 80,000 | 100,000 | 120,000 |
| 2,500 sq ft | 100,000 | 120,000 | 160,000 |
| 3,000 sq ft | 120,000 | 140,000 | 180,000 |
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
Bryant's rule of thumb puts it at 60,000 to 120,000 BTU depending on climate. In climate zone 4 with average insulation, 2,000 sq ft needs about 95,000 BTU/h of heat, which is a 100,000 BTU furnace at 95% efficiency.
How many BTUs per square foot do I need to heat a house?
About 30–35 BTU per sq ft in climate zone 1, 35–40 in zone 2, 40–45 in zone 3, 45–50 in zone 4 and 50–60 in zone 5. Bryant's guidance matches at the ends — 30–35 in warm regions, 50–60 in cold ones.
What about climate zones 6, 7 and 8?
We haven't found a published per-square-foot figure for them, so the calculator uses zone 5's 50–60 BTU. In those zones treat the result as a floor and pick the poor-insulation setting unless the house is tight; a Manual J calculation matters most here.
Which climate zone am I in?
The zones run from 1 in southern Florida and Hawaii to 7 and 8 in northern Minnesota and Alaska. Look up your county on the IECC climate zone map that the Department of Energy and ENERGY STAR publish; it's the same map the insulation calculator uses.
What's the difference between input and output BTU?
Furnaces are labeled by input, the fuel they burn. Output is input × AFUE efficiency. A 100,000 BTU furnace delivers about 95,000 BTU/h of heat at 95% AFUE, or 80,000 BTU/h at 80%.
Is a bigger furnace better?
No. An oversized furnace heats the house in short bursts and shuts off, which feels less even and wears parts faster. Aim for the smallest size that covers your heat need.
What is a Manual J calculation?
The industry-standard room-by-room heat loss calculation (from ACCA) that accounts for insulation, windows, air leaks and local design temperatures. Bryant notes that the square-foot method is only an estimate and recommends a professional load calculation.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.