Furnace sizing calculator

What size furnace do I need by square footage?

Furnace sizing follows the same logic as heat pump sizing: heating load at the winter design temperature drives the answer, not square footage alone. Most homes in a mixed climate need 25 to 35 BTU of output per square foot. In cold climates that jumps to 45 to 60 BTU per sq ft. Quick reference for a typical mixed climate (Zone 4) with average insulation, sized to output BTU at the 99% design temperature:

• 1,000 sq ft: 40,000 to 60,000 BTU input, 35,000 to 55,000 BTU output
• 1,500 sq ft: 60,000 to 80,000 BTU input
• 2,000 sq ft: 80,000 to 100,000 BTU input
• 2,500 sq ft: 100,000 to 120,000 BTU input
• 3,000 sq ft: 120,000 to 140,000 BTU input

Furnaces are sold by input BTU rating, which is what shows on the data plate. The output rating is the BTU per hour the furnace actually delivers into your house, which is input times AFUE. An 80,000 BTU input furnace at 96% AFUE delivers 76,800 BTU output. An 80,000 BTU input furnace at 80% AFUE only delivers 64,000 BTU output. Same nameplate, different real heating.

How AFUE changes the furnace size you actually need

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel energy the furnace converts to useful heat. The rest goes up the flue. Higher AFUE means more heat for your money but also smaller equivalent input rating. If your home needs 60,000 BTU/hr output to stay warm on a design day, the furnace input you should shop for varies by AFUE tier:

• 80% AFUE standard: 60,000 ÷ 0.80 = 75,000 BTU input (round up to 80,000 standard)
• 92% AFUE mid-efficiency: 60,000 ÷ 0.92 = 65,000 BTU input (round up to 80,000)
• 96% AFUE high-efficiency condensing: 60,000 ÷ 0.96 = 63,000 BTU input (round up to 80,000)
• 98% AFUE modulating: 60,000 ÷ 0.98 = 61,000 BTU input (round up to 80,000)

Notice they all round up to the same standard 80,000 BTU class. Furnaces only come in 40,000, 60,000, 80,000, 100,000, 120,000, and 140,000 BTU sizes. The AFUE doesn't change your shopping size much. What it does change is fuel consumption: that same 80,000 BTU input furnace at 96% AFUE burns 20 percent less gas than the 80% AFUE version to deliver the same heat.

Natural gas vs propane vs electric furnace sizing

The output BTU rating you need stays the same regardless of fuel. What changes is the cost per BTU and the annual fuel use. Comparison for a 60,000 BTU/hr output furnace running through a typical mixed climate winter:

• Natural gas (96% AFUE, $1.35/therm): 540 therms/year, $730 annual fuel cost
• Propane (96% AFUE, $2.80/gallon): 590 gallons/year, $1,650 annual cost
• Heating oil (85% AFUE, $4.10/gallon): 460 gallons/year, $1,885 annual cost
• Electric resistance (100% efficient, $0.18/kWh): 15,800 kWh/year, $2,845 annual cost

Natural gas is the cheapest by a wide margin where it's available. Propane and oil are similar mid-cost options for rural areas without gas service. Electric resistance is the most expensive way to heat by far, which is why heat pumps replaced electric furnaces in nearly every cold-climate state that gets electric service. If you're considering electric resistance heat, run the numbers against a heat pump first.

Furnace installation cost by AFUE tier

What you should expect to pay an HVAC contractor for a furnace install, fully done. Based on current HomeGuide, Modernize, and Angi data for mid-tier brands like Carrier, Trane, Lennox, Goodman, and Rheem.

• 80% AFUE standard gas furnace: $3,800 to $6,500 installed
• 92% AFUE mid-efficiency gas furnace: $4,500 to $7,500 installed
• 96% AFUE high-efficiency condensing gas furnace: $5,500 to $9,000 installed
• 98% AFUE modulating gas furnace: $7,000 to $11,000 installed
• Electric resistance furnace: $2,000 to $4,500 installed (cheapest install)
• Oil furnace: $4,500 to $9,500 installed

The jump from 80% to 96% AFUE costs $1,500 to $2,500 more upfront. In a mixed climate at $1.35/therm gas, that pays back in 4 to 7 winters. In a cold climate at the same gas rate, payback drops to 2 to 4 winters. The 96% AFUE upgrade is the no-brainer. The 98% modulating upgrade only pays back in very cold climates or homes with above-average heating loads.

Why oversizing a furnace costs you money and comfort

A furnace that is too big for the house cycles on and off too fast. The burners light, the blower kicks in, the thermostat reaches setpoint in 3 to 5 minutes, the burners shut off, and the blower drops out. The house cools below setpoint within 10 minutes, and the cycle repeats. This is called short-cycling, and it produces three problems: temperature swings that feel uncomfortable, premature wear on the ignition and gas valve from repeated starts, and lower real-world AFUE because the furnace never reaches steady-state combustion temperature.

A correctly sized furnace runs for 15 to 25 minutes on a typical winter day and 30 to 45 minutes on a cold day. That long run time pulls humidity out via the flue, keeps the heat exchanger above condensation temperature in 80% AFUE units, and gives the blower time to distribute heat evenly through the ducts. ACCA Manual S says to size for the design-day load plus 10 percent. The calculator above bakes that 10 percent margin in.

Carrier, Trane, Lennox, Goodman: which furnace brand to buy

All gas furnace brands meet the same federal AFUE minimums and use similar heat exchanger designs at the same price tier. The differences come down to warranty, dealer network, and controls. Carrier Infinity, Trane XV, and Lennox SLP98V are the top-tier modulating variable-speed units, all priced in the same $7,000 to $11,000 installed range. Goodman GMVM and Rheem RGFC sit in the value tier with similar AFUE and shorter heat exchanger warranties at $5,500 to $7,500 installed. American Standard is the Trane-built value brand at slightly lower prices than Trane itself.

For a standard 80% or 92% AFUE furnace, brand barely matters. Goodman and Rheem give you the best price per BTU and Carrier or Trane give you the longest dealer network. For a premium modulating 98% AFUE furnace, the Carrier Infinity and Trane XV are worth the premium for the smoother modulation and quieter operation. Either way, the right install matters more than the brand. Get three quotes from licensed HVAC contractors and pick the one who runs a Manual J in writing.

When to choose a heat pump instead of a new furnace

Before you sign a quote on a new gas furnace, run the heat pump vs gas furnace calculator on this site with your specific utility rates. In most U.S. climates with electricity under $0.20 per kWh and natural gas above $1.20 per therm, a cold-climate heat pump beats a new high-efficiency gas furnace on 15-year total cost. In very cold climates with cheap gas, the furnace wins. Dual fuel systems that run a heat pump above 30°F and a gas furnace below that often beat both options. The sizing calculator on this page gets you the furnace answer. The other tool gets you the comparison.