How the BTU calculator works
The baseline and the adjustments
The DOE-era rule of thumb starts at 20 BTU per square foot of floor area. Real rooms then bend that number: tall ceilings add volume (we scale by ceiling ÷ 8), sun-loaded rooms add ~10%, shaded ones save ~10%, each occupant beyond two adds ~600 BTU of body heat, and kitchens add ~4,000 BTU for cooking load. Hot-humid climates add a final ~10%.
Bigger is not better — it's wetter
Oversized ACs are the industry's most common mistake. An oversized unit cools the air fast, shuts off before dehumidifying it, and leaves the classic oversize signature: cold but clammy, short-cycling, higher bills, shorter equipment life. Right-sized is the goal — which is why the calculator adjusts rather than pads.
From BTUs to units
- Window units: sold in 5,000–25,000 BTU steps; buy the nearest size at or slightly below your number.
- Mini-splits: sold by the ton (1 ton = 12,000 BTU); a 9,000 BTU head covers most bedrooms, 12,000 most living rooms.
- Whole-house central: stop here and get a professional Manual J — duct losses, insulation, and envelope tightness move the answer too much for rules of thumb.