BTU SizerHeating size calculators
Heating is where the climate zone stops being a footnote. Zone 7 asks for twice what zone 1 does for the same room.
Radiator size calculator
BTU from room volume — and the delta-T that quietly halves it.
Baseboard heater size calculator
Watts converted into feet of heater you can actually measure on the wall.
Furnace size calculator
Output BTU by climate zone — then the AFUE step everyone forgets.
Garage heater size calculator
The coldest room in the house, sized honestly — door and slab included.
Heat pump size calculator
Sized on the heating load — and checked at the temperature that matters.
Space heater watts calculator
Watts for the room — and why 1,500 is the ceiling on a normal plug.
Heating sizing has one input that cooling does not, and it dominates everything else: how cold it gets where you live. The load is proportional to the difference between the temperature you want inside and the temperature the system has to work against outside, and that difference ranges from about 25 °F on the Gulf coast to 70 °F in northern Minnesota.
That is the whole reason the whole-system calculators here — furnace, heat pump, garage heater — ask for a climate zone before anything else. Everything after it — ceiling height, insulation, the room’s shape — moves the answer by ten or fifteen percent. The zone moves it by a factor of two.
Frequently asked questions
Why does heating need a climate zone when cooling does not?
Because the temperature difference the system has to bridge varies far more in winter. Cooling works against maybe 20 °F between inside and outside almost everywhere. Heating in Miami works against 25 °F and heating in Duluth against 70 °F, and the load is proportional to that gap. That is why one national BTU-per-square-foot number works for cooling and never for heating.
How many BTU per square foot do I need for heating?
From about 27 BTU/h per square foot in the warmest zone to 57 in the coldest, for an average-insulation house. A well-sealed modern build sits at the bottom of its zone’s range and an uninsulated older one above the top of it, which is why insulation is a separate correction rather than being folded into the rate.
Is an oversized furnace as bad as an oversized air conditioner?
It is less harmful but still wrong. A furnace has no dehumidification job to fail at, so the clammy-room problem does not appear. What you get instead is short cycling, uneven temperatures between the start and end of a cycle, a noisier house, and more wear on the ignition and blower. Twenty to thirty percent above the calculated load is the usual allowance; double is not.
Do electric heaters use a different rule?
They use watts rather than BTU, but the arithmetic is the same load divided by 3.412. The convention for electric resistance heat is about 10 watts per square foot for an average room, which is 34 BTU/h per square foot — a zone 3 to 4 figure. Cold-zone rooms and poorly insulated ones need 12 to 15 watts per square foot.
How we calculate this
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