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Heating BTU calculator
1,800 sq ft with average insulation in zone 3, the setup the form opens on, needs 77,400 BTU/h of heat output. That is a furnace rated 81,000 BTU input at 96% AFUE, or 97,000 BTU at 80%. Heating is the bigger load in most of the US, and the input-vs-output trap on furnace spec sheets is where it costs people money, so this page gives you output first and then the input rating you actually shop for.
BTU per square foot by climate zone
The classic heating rule of thumb runs from about 30–35 BTU/sq ft along the Gulf to 50–60 in the far North; this calculator uses the midpoints (33 / 38 / 43 / 48 / 55) and then swings ±20% for the envelope. Zones follow the familiar five-band US heating map; if you're on a border, split the difference or size to the colder band.
Reading a furnace spec sheet
Furnaces are labeled by input; multiply by AFUE for output. The calculator's two input rows bracket the market: 96%+ condensing models and 80% conventional ones. Heat pumps are labeled in tons like AC but their output falls with the outdoor temperature, so in zones 3–5 look for the rated capacity at 5°F (cold-climate models publish it) and expect a backup heat source for the deepest cold. To size a heat pump to the larger of your heating and cooling loads, use the heat pump size calculator.
Same caveat as cooling: this is a rule-of-thumb load, not a Manual J. Oversized furnaces short-cycle just like oversized AC: comfort and efficiency both suffer.
Frequently asked questions
How many BTU do I need to heat 1,500 sq ft?
Between roughly 50,000 and 90,000 BTU/h of heat output depending on winter climate and insulation: about 50,000 in the South, 65,000 in a mixed climate, and up to 90,000+ for an older house in the far North. Enter your numbers above for the specific answer.
Is furnace input BTU the same as output BTU?
No, furnaces are advertised by input. An 80,000 BTU input furnace at 96% AFUE delivers ~76,800 BTU of heat; at 80% AFUE only 64,000. This calculator gives you the required output, plus the matching input at both common efficiencies, so you can read a spec sheet without being fooled.
Can a heat pump heat my house in a cold climate?
Modern cold-climate heat pumps hold useful capacity well below 0°F, and they're now the standard recommendation in most of the country. The catch is that capacity falls as temperature drops, so they're sized to a "balance point" and often paired with electric or furnace backup for the coldest snaps. Check a model's capacity at 5°F, not its nameplate.
Why does insulation change the answer so much?
Heating load is heat loss: every BTU that leaks out through the attic, walls and gaps must be replaced. A tight modern envelope loses 20% less than average; an older leaky one loses 20% more. That ±40% swing is bigger than most climate-zone differences, which is why air-sealing before buying equipment often pays for itself.