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.