BTU sizing guide: matching cooling power to real rooms

The room-unit baseline is simple: about 20 BTU per square foot of floor area — 5,000–6,000 BTU to ~250 sq ft, 8,000 to ~350, 10,000 to ~450, 12,000 to ~550, 14,000 to ~700 (see our size-by-size pages for the worked table). Then apply the adjustments the industry standardized around ENERGY STAR guidance:

• very sunny room: +10% · heavily shaded: −10%;
• kitchen: +4,000 BTU (appliances are heat sources);
• more than two regular occupants: +600 BTU per person;
• poor insulation, top floor under a hot roof, or big west glass: size toward the next step up;
• ceilings well above 8 ft: think in volume — add roughly 10% per extra foot.

Why oversizing backfires

An oversized unit slams the thermostat down fast and shuts off before removing moisture: the room reads 72 °F and feels like a cave — clammy, cold-walled, cycling loudly. Right-sized equipment runs longer, gentler cycles that wring humidity out; in muggy climates this is the difference between comfort and complaint. When in doubt between two sizes for a humid-summer room, the smaller unit with an inverter compressor usually wins the comfort contest.

When square-footage math is not enough

Whole-house equipment — central AC, heat pumps, multi-zone ductless — deserves a Manual J load calculation (the ACCA standard): room-by-room accounting of walls, windows, orientation, infiltration and internal gains. Contractors who size by « square feet per ton » folklore routinely land a full ton off in either direction. For a single window or portable unit, the table plus adjustments above is honestly sufficient.

Frequently asked questions

How do BTU, tons and kW relate?

12,000 BTU/h = 1 ton = 3.517 kW of cooling. A « 3-ton » central system is 36,000 BTU/h; a European « 2.5 kW » split is about 8,500 BTU/h.

Can one big unit cool two rooms?

Through an open double doorway, partially; through a standard door or hallway, no — air throw dies at corners. Two small units beat one big one across separated rooms every time.

Does climate change the per-square-foot rule?

At the room-unit scale the adjustments absorb most of it (sun, insulation). At whole-house scale, absolutely — design temperatures drive Manual J, which is exactly why the professional calculation exists.

More United States guides: SEER2 explained: what the rating means on your bill · Heat pump vs air conditioner: the real decision framework · Central AC replacement cost: an honest 2026 breakdown · Mini-split vs central air: which architecture fits your house · HVAC tax credits and rebates: claiming what you are owed.