Furnace Size Calculator: What Size Furnace Do I Need?

Your details

The total floor area you want to heat. Include finished basements but use 50% of basement area if it is partially below grade. Exclude unheated garages and attics.
sq ft
Pick the zone that matches where you live. Colder climates need more BTU per square foot. If you are between zones, round up.
Poor insulation adds about 15% to the required BTU; good insulation cuts it by about 10%. When in doubt, use Average.
Standard homes have 8 ft ceilings. Each foot above 8 ft adds about 12.5% to the heating load because there is more air volume to heat.
ft
Annual Fuel Utilization Efficiency. High-efficiency condensing furnaces are 90-98%; mid-efficiency are 80-89%; older units can be 60-80%. The calculator uses this to determine how large the input BTU rating needs to be to deliver the required output BTU.
%
Maximum output BTUVery large / commercial-grade
110,000BTU/h

Recommended furnace output capacity - use this as your target size

Minimum output BTU90,000BTU/h
Required input BTU rating114,583BTU/h
Furnace size in tons9.2tons
BTU per sq ft used55BTU/sq ft
110,000 BTU/h
Small (< 60K BTU)<60000Medium (60-100K BTU)60000-100000Large (100-150K BTU)100000-150000Very Large (>150K BTU)150000+

Size your furnace at 110K BTU output (9.2 tons).

  • Your home needs between 90,000 and 110,000 BTU/h of heating output based on Zone 4 - Cool and average insulation.
  • At 96% AFUE, you need a furnace with an input rating of at least 114,583 BTU/h on the nameplate.
  • Larger furnaces benefit most from variable-speed blowers and modulating burners to avoid short-cycling on mild days.
  • Slightly oversizing (by 10-15%) is acceptable; grossly oversizing causes short-cycling, uneven temperatures, and excess humidity.

Next stepTypical cities in Zone 4 - Cool: Chicago, Denver, Philadelphia. Have a licensed HVAC contractor run a full Manual J load calculation before purchasing a furnace to confirm this estimate.

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