CalcMyHVAC

What size furnace do you need?

Work out how much heat your home needs, then which furnace delivers it at your efficiency rating.

Size of your home
sq ft
Location

Enter a ZIP code and we set the climate zone for you.

Home and furnace
ft

How to size a furnace

  1. Find the heat your home needs (output BTU): square footage × the BTU per square foot for your climate zone, adjusted for ceiling height and insulation.
  2. Divide by the furnace's AFUE to get the input rating, the number in the model name.
  3. Pick the nearest standard size: 40,000, 60,000, 80,000, 100,000 or 120,000 BTU input.

Furnace input BTU = (sq ft × zone BTU per sq ft) ÷ AFUE

Heating BTU per square foot by climate zone

ZoneTypical areaBTU per sq ft
Zone 1 — HotFlorida, Hawaii, far south Texas32.5
Zone 2 — WarmTexas, Arizona, Gulf states37.5
Zone 3 — MildCalifornia, the Carolinas, Tennessee42.5
Zone 4 — CoolMid-Atlantic, Midwest south, Pacific NW47.5
Zone 5 — ColdNew England, upper Midwest, Mountain north55

Furnace size chart

Heat output needed for an average home with 8-foot ceilings.

Home sizeWarm climateMild climateCold climate
1,000 sq ft32,50042,50055,000
1,200 sq ft39,00051,00066,000
1,500 sq ft48,75063,75082,500
1,800 sq ft58,50076,50099,000
2,000 sq ft65,00085,000110,000
2,500 sq ft81,250106,250137,500
3,000 sq ft97,500127,500165,000

Considering a heat pump instead? In zones 1–3 a heat pump can usually replace a furnace entirely. Try the heat pump size calculator.

Common questions

What size furnace do I need for a 2,000 sq ft house?

Roughly 65,000 BTU of heat output in a warm southern climate, 85,000 in a mild climate and 110,000 in a cold northern climate. With a 95% efficient furnace, that means an 80,000, 100,000 or 120,000 BTU input model.

What is the difference between furnace input and output BTU?

Input BTU is the fuel the furnace burns; output BTU is the heat delivered to your home. Output equals input times AFUE. An 80,000 BTU input furnace at 95% AFUE delivers about 76,000 BTU of heat.

What is AFUE?

Annual Fuel Utilization Efficiency — the share of fuel that becomes useful heat. Standard gas furnaces are 80% AFUE; high-efficiency condensing furnaces are 90–98%.

Can I size a furnace by ZIP code?

Yes. Enter your ZIP code above and the calculator sets your climate zone, which changes the BTU per square foot used for heating.

Is it bad to oversize a furnace?

An oversized furnace short-cycles, which causes uneven temperatures, more wear on the heat exchanger and higher fuel bills. Size to the load, not the biggest unit that fits the budget.