Furnace and heating BTU planning starts with heated square footage, insulation, and climate. Enter the zone you want warm — we estimate heating load for DIY comparison before you talk to a HVAC pro.
Cooling load removes heat and moisture; heating load replaces heat lost through walls, windows, and air leaks. Cold climates and leaky shells need more heating BTU per sq ft than the cooling number for the same room.
Use this page for furnace, boiler, or electric heat planning. For ductless winter performance, pair results with the heat pump cold climate calculator.
A 1,200 sq ft ranch with average insulation in a cold climate often lands near 40,000–60,000 BTU input for whole-home planning — layout and windows swing that range. A finished basement zone of 400 sq ft may need its own 15,000–25,000 BTU of capacity depending on below-grade losses.
Garages and workshops are worse: large doors and thin walls dominate. Size those with the garage heater BTU calculator instead of a living-room factor.
Furnace nameplates list input BTU and AFUE. Output ≈ input × AFUE. An 80,000 BTU input furnace at 95% AFUE delivers about 76,000 BTU of heat — shop output against your load, not marketing input alone.
This tool is a DIY planning estimate. Permits, gas-line sizing, and final equipment selection need Manual J / Manual S from a licensed contractor.
Mild climates often run heat pumps as primary heat. Cold-climate models still need honest design temperatures — see heat pump vs air conditioner before you rip out a working furnace.
Cold-climate planning often lands near 40,000–60,000 BTU of heating capacity depending on insulation and windows — not a fixed rule. Enter your layout here, then confirm with Manual J before buying equipment.
No. Output ≈ input × AFUE. An 80,000 BTU input furnace at 95% AFUE delivers about 76,000 BTU of heat. Match output to load.
Often in mild climates. Cold-climate heat pumps need capacity at your design temperature — use the heat pump cold climate calculator and talk to an installer about dual fuel or backup heat.
HVAC Calculators · https://hvaccalculators.net/furnace-btu-calculator/