BTU calculator: size heating and cooling by room dimensions, insulation, and climate to get the right BTU output. Free, runs locally, no signup required.
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Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.
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BTU calculator: size heating and cooling by room dimensions, insulation, and climate to get the right BTU output. Free, runs locally, no signup required.
The 2,728 BTU/hr estimate breaks into base load, sun, and people.
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
Formula
Base load is about 18-20 BTU/hr per square foot of floor area, adjusted upward for occupants (about +600 each) and climate.
Worked example
A 15 x 12 ft room (180 sq ft) with 2 occupants needs about 3,160 BTU/hr of heating or cooling capacity.
BoringToolsKit. “BTU Calculator.” boringtoolskit.com/btu-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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A worked example: 12 by 10 by 8 foot bedroom
Take a bedroom 12 ft long, 10 ft wide, with an 8 ft ceiling. Volume is 12 x 10 x 8 = 960 cubic feet. The base rule of 1.5 BTU per cubic foot gives 960 x 1.5 = 1,440 BTU/hr before any adjustments.
Now suppose the room gets strong afternoon sun and usually holds two people. Sun exposure adds 20% of the base: 1,440 x 0.20 = 288 BTU/hr. Occupancy adds 500 BTU per person: 2 x 500 = 1,000 BTU/hr. The total is 1,440 + 288 + 1,000 = 2,728 BTU/hr.
Dividing across the 120 sq ft floor gives about 22.7 BTU per sq ft, roughly in line with typical cooling loads of 20 BTU per sq ft for insulated rooms. In practice you would buy the next common size up, a 5,000 BTU window unit, since 2,728 is the minimum, not a target. The same room with a 9 ft ceiling instead of 8 would need 12 x 10 x 9 = 1,080 cubic feet, or 1,620 BTU base: 180 BTU/hr more for a single extra foot of height.
Where this estimate breaks down
The 1.5 BTU per cubic foot rule assumes average US insulation and a moderate climate, so it misses rooms that sit far from that average. An uninsulated garage or an attic room above a hot roof can need 2 to 3 times the estimate, while a well-sealed basement may need less than half. Window area is also absent from the math: a single 3 x 4 ft south-facing window can add several hundred BTU/hr of load on a summer afternoon, which matters more in a 960 cubic foot bedroom than in an open plan living area.
Heating flips the arithmetic. Cold-climate heating rules of thumb run 30 to 60 BTU per sq ft, roughly double the cooling figure, and the sun adjustment that adds load in July subtracts it in January. Kitchens add their own load: an oven at 350 F can dump 3,000 BTU/hr or more into the room, swamping the 500 BTU per person assumption. For anything beyond a single room, or for sizing central equipment, a Manual J load calculation done with wall, window, and insulation data is the real tool. Treat this calculator as a screening estimate that gets you into the right product size bracket.
