Heating BTU Calculator
Estimates the heat a space needs on the coldest day of the year. Use it to size a boiler, a heat pump, a stove or electric heating.
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The formula
BTU/h = floor area (ft²) × climate factor × (ceiling height ÷ 8 ft) × fabric factor kW = BTU/h ÷ 3,412
Heat loss is driven by the temperature difference between inside and out, so a colder climate needs a bigger factor for the same house. The fabric factor stands in for how fast that heat escapes — insulation, glazing and draughts — which is the one thing you can actually change.
What to watch out for
Two things are worth understanding before you size anything off this:
- This is the design load, not the average. It is what the house needs at the worst winter temperature. On a normal day the heating runs well below it. Sizing on the design load and then adding a large margin is how houses end up with boilers that short-cycle all winter.
- Insulation beats plant size. Moving from poor to good fabric cuts the load by more than 40% in this model, and roughly the same in reality. It is the only change that reduces the bill every year rather than moving it around.
For a heat pump in particular, a proper room-by-room heat loss survey is worth paying for — the emitters have to match the low flow temperature, and a whole-house figure cannot tell you that.
Frequently Asked Questions
How many BTU do I need to heat a room?
Around 35 BTU/h per square foot in a cool climate with average insulation, so a 150 ft² bedroom needs roughly 5,300 BTU/h. Colder climates and poor insulation push that toward 60–85 BTU/h per square foot.
How do I convert BTU to kW?
Divide by 3,412. A 24,000 BTU/h load is about 7 kW. Going the other way, multiply kilowatts by 3,412.
Is it bad to oversize a boiler?
Yes. An oversized boiler reaches temperature quickly and shuts down, then restarts a few minutes later. That cycling wastes fuel, wears the components and stops a condensing boiler from condensing, which is where its efficiency comes from.