Heating BTU Calculator
Size a furnace or heater. Enter the room dimensions, your climate zone, and insulation quality to estimate the heating BTUs and a furnace size to look for.
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Get quotesClimate drives heating load
Heating needs depend heavily on where you live: a mild climate might need ~30 BTU per square foot while a very cold one needs ~60. Insulation, ceiling height, and air leakage adjust that further. Because furnaces aren't 100% efficient, the input rating you shop for is higher than the heat delivered — the furnace-size line accounts for that. For a whole house, a professional Manual J load calculation is the gold standard.
How it’s calculated
Heating BTU ≈ area × climate factor × insulation factor × (ceiling ÷ 8). Furnace input size = BTU ÷ efficiency (AFUE).
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Worked example
A 20x15 room in a moderate climate needs about 12,000 heating BTU, or a ~15,000-BTU furnace input at 80% efficiency.
Common mistakes
- Treating this as a substitute for a Manual J load calc.
- Ignoring insulation and ceiling height.
Where it is used
- Ballparking a furnace or heater size.
- Sanity-checking an HVAC quote.
Frequently asked questions
Is this a substitute for Manual J?
No — it's a quick estimate. For equipment selection, an HVAC pro's Manual J load calculation accounts for windows, orientation, and air sealing.
Why is furnace size higher than BTUs needed?
Furnaces lose some energy; an 80% AFUE unit delivers 80% of its rated input as heat, so you size up to cover the loss.
Does ceiling height matter?
Yes — taller rooms hold more air to heat, so the load scales with height above the standard 8 feet.
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