Estimate the heating BTU/hr output needed and the required furnace input rating from floor area, U.S. climate zone, insulation quality and AFUE.
Enter the floor area, climate zone and insulation to size the heating output and furnace.
Planning estimate only — confirm with a full ACCA Manual-J load calculation.
About this tool
The Furnace / Heating BTU Size Calculator is a free, in-browser HVAC tool that estimates the heating output a home needs and the furnace input rating required to deliver it. The required output BTU/hr is floor area (ft²) multiplied by a climate-zone heating factor — roughly 30 BTU/hr·ft² in the hot southern Zone 1, rising through 35, 40 and 45 to about 55–60 in the cold northern zones — then scaled by an insulation modifier and by ceiling height ÷ 8.
All of the arithmetic happens locally in your browser, so nothing you type leaves your device. The insulation modifier is 0.9 for a well-insulated, newer home, 1.0 for average construction, and 1.2 for a poorly insulated or older house. To size the appliance itself, the required output is divided by the furnace's AFUE efficiency (default 95%, editable) to get the input rating, and the tool suggests the nearest standard furnace input — 40k, 60k, 80k, 100k, 120k or 140k BTU.
Results show the required heating output, the furnace input rating, the suggested standard furnace size, and the heating intensity in BTU per square foot for a quick sanity check. Like any rule-of-thumb method this is a planning estimate; a full ACCA Manual-J that accounts for windows, air leakage and duct losses should confirm the final furnace selection.
Frequently asked questions
How many heating BTU per square foot do I need?
It depends on climate: about 30 BTU/hr·ft² in hot southern zones up to 55–60 in cold northern zones, adjusted for insulation and ceiling height. A 1,500 ft² home in a mid climate (40 BTU/ft²) needs roughly 60,000 BTU/hr of output.
What is the difference between furnace input and output?
Output is the usable heat delivered to the home; input is the fuel energy the furnace consumes. They are related by efficiency: input = output ÷ AFUE. A 95% AFUE furnace delivering 60,000 BTU/hr output draws about 63,200 BTU/hr input.
What is AFUE?
Annual Fuel Utilization Efficiency is the percentage of fuel energy a furnace turns into usable heat over a season. Modern condensing furnaces are 90–98% AFUE; older units can be 60–80%. Higher AFUE means a smaller input rating for the same output.
What size furnace do I need?
Take the required output, divide by AFUE for the input, and pick the nearest standard size at or above it — 40k, 60k, 80k, 100k, 120k or 140k BTU. Oversizing causes short-cycling, so a Manual-J load calc should confirm the choice.