Furnace BTU Calculator

Calculate furnace BTU for your home. Adjust for climate zone, insulation, ceiling height, AFUE efficiency, and fuel type. See estimated annual heating cost.

🔥 Furnace BTU Calculator

Home Size (sq ft)

Climate Zone

Insulation

Ceiling Height

Furnace Efficiency

Fuel Type

Fuel Cost ($/therms)

HOME

Home Size1,500.00 sq ft

Ceiling Height8 ft

BTU SIZING

BTU Output Needed52,500.00 BTU/hr

Equivalent kW15.4 kW

FURNACE & COST

Furnace Input (95% AFUE)55,263.16 BTU/hr

Est. Annual Fuel497 therms

Est. Annual Cost$597

💡 How to Size a Furnace

A furnace's BTU (British Thermal Unit) rating determines how much heat it can produce per hour. Choosing the right size is critical — too small and your home stays cold on the worst days; too large and the furnace short-cycles, wasting fuel and wearing out faster.

The calculator above uses climate zone BTU factors, adjusts for insulation, ceiling height, and AFUE efficiency, and estimates annual heating cost across 4 fuel types (gas, propane, oil, electric).

Calculate airflow with our CFM calculator. For cooling, see our window AC size calculator. Measure room area with our square footage calculator.

BTU per Square Foot by Climate Zone

The DOE and HVAC industry recommend different BTU/sq ft based on your heating climate:

Climate Zone	BTU/sq ft	States
Mild	25–30	FL, TX, AZ, GA, LA, SC, MS, AL, NM
Moderate	30–40	NC, VA, TN, KY, MO, KS, OK, AR, DE, MD
Cold	40–50	NY, PA, OH, MI, IL, IN, NJ, CT, MA, WI, IA
Very Cold	50–60	MN, ND, SD, MT, WY, VT, NH, ME, AK
Subarctic	60+	Interior AK, mountain regions

Example: 2,000 sq ft home in cold climate: 2,000 × 45 = 90,000 BTU output needed.

Insulation Adjustment

Insulation	Adjustment	Description
Poor	+30%	Old home, single-pane windows, no wall insulation, drafty
Average	Baseline	Standard double-pane, R-19 attic, some wall insulation
Good	−15%	Updated insulation, tight envelope, energy-efficient windows
Excellent	−30%	New construction, spray foam, triple-pane, air-sealed

An energy audit ($200–$500) can identify where your home loses heat most. Adding attic insulation is often the highest-ROI upgrade.

AFUE Efficiency Explained

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel converted to heat. The rest goes up the flue.

AFUE	Type	Heat from 100K BTU Input	Avg Cost
80%	Standard	80,000 BTU	$1,500–$3,000
90%	Mid-Efficiency	90,000 BTU	$2,500–$4,000
95%	High-Efficiency	95,000 BTU	$3,000–$5,000
98%	Condensing	98,000 BTU	$4,000–$6,500

In cold climates, a 95%+ AFUE furnace pays for itself in 3–5 years through fuel savings. In mild climates, an 80% AFUE may be more cost-effective.

Fuel Type Comparison

Fuel	BTU/Unit	Avg Cost	Annual Cost (90K BTU home)
Natural Gas	100,000/therm	$1.00–$1.50/therm	$600–$1,200
Propane	91,500/gallon	$2.50–$3.50/gal	$1,800–$3,000
Heating Oil	138,500/gallon	$3.00–$4.50/gal	$1,500–$2,800
Electric	3,412/kWh	$0.10–$0.25/kWh	$1,500–$4,000

Natural gas is the cheapest heating fuel in most US markets. Propane and oil are common in rural areas without gas lines. Electric furnaces are nearly 100% efficient but expensive to run.

Common Furnace Sizes

Furnace (Input BTU)	Output (95% AFUE)	Home Size (Moderate)
40,000	38,000	800–1,100 sq ft
60,000	57,000	1,100–1,600 sq ft
80,000	76,000	1,600–2,200 sq ft
100,000	95,000	2,200–2,700 sq ft
120,000	114,000	2,700–3,300 sq ft

1,500 sq ft, moderate climate, average insulation, 95% AFUE gas: 52,500 BTU output → 55,263 BTU furnace input. Est. annual cost: ~$630.