A sizing estimate for air conditioning and heating, adjusted for ceiling height, climate, insulation, sun exposure and occupancy.
A British thermal unit is the energy needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour — the rate at which they move or produce heat. One ton of cooling is 12,000 BTU/hr, a unit that dates from how much heat it takes to melt a ton of ice in a day. In metric markets the same thing is quoted in kilowatts: 1 kW is about 3,412 BTU/hr.
The starting point for cooling is roughly 20 BTU per square foot, assuming an eight-foot ceiling. A 500 sq ft room therefore needs about 10,000 BTU/hr, or 0.83 tons. Heating is more variable because it depends on how cold it actually gets outside — anywhere from 30 BTU per square foot in a mild climate to 60 in a severe one.
Everything else on this page adjusts that baseline. Ceiling height scales it directly, because you are conditioning a volume rather than a floor. Poor insulation raises it; good insulation lowers it. Large sun-facing windows add load. Each person beyond two adds roughly 600 BTU/hr, and a kitchen adds about 4,000 for the cooker.
This is the single most useful thing to know about air conditioner sizing, and it is counter-intuitive enough that it gets ignored constantly.
An oversized air conditioner cools the air to the thermostat setpoint very quickly and then shuts off. That sounds efficient and is not, for two reasons. Cooling a room also means dehumidifying it, and that happens as moisture condenses on the cold evaporator coil — which takes sustained runtime. A unit that runs in short bursts hits the temperature target while leaving the room damp, producing that clammy, cold-but-uncomfortable feeling. Second, compressors draw a large surge at start-up, so frequent short cycling uses more energy and wears the compressor faster than longer, steadier runs.
A correctly sized unit runs for long stretches on a hot day. That is what it is supposed to do.
| Room area | Cooling (BTU/hr) | Approx. kW |
|---|---|---|
| 150 sq ft (14 m²) | 5,000 | 1.5 |
| 250 sq ft (23 m²) | 6,000 | 1.8 |
| 350 sq ft (33 m²) | 8,000 | 2.3 |
| 500 sq ft (46 m²) | 10,000 | 2.9 |
| 700 sq ft (65 m²) | 14,000 | 4.1 |
| 1,000 sq ft (93 m²) | 18,000–20,000 | 5.3–5.9 |
| 1,500 sq ft (139 m²) | 24,000–30,000 | 7.0–8.8 |
You may notice the calculator returns a smaller figure than this table for small rooms — 3,000 BTU/hr for 150 sq ft against the 5,000 shown above. Both are right, and the gap is worth understanding. The calculator reports the actual computed load; the table reflects the units people actually buy, and the smallest window air conditioner generally sold is 5,000 BTU. Below about 250 sq ft you are choosing the smallest available unit rather than matching a calculated load, which is one of the few situations where mild oversizing is unavoidable.
In India and much of Asia air conditioners are sold in tons rather than BTU — a 1 ton unit is 12,000 BTU/hr and suits roughly a 120 to 150 sq ft bedroom; 1.5 ton covers around 180 to 200 sq ft.
Professionals size systems with a Manual J load calculation, which accounts for the actual construction of each wall, the specific window glazing and orientation, air infiltration rates measured or estimated, duct losses, and local design temperatures. It routinely produces answers well below the square-foot rule of thumb, which is one reason so much installed equipment is oversized.
Use the figure here to sanity-check a quote or narrow a shopping list. For a whole-house system, ducted installation or heat pump, get the real calculation — the equipment cost difference between sizes is small next to years of running an oversized system badly.
About 20 BTU per square foot for cooling with an eight-foot ceiling, as a starting point. Heating ranges from about 30 BTU per square foot in a mild climate to 60 in a severe one. Adjust upward for high ceilings, poor insulation, large sun-facing windows, a kitchen, or a room that is usually crowded — and downward for a well-insulated modern build or a heavily shaded room.
No, and this is the mistake that causes most comfort complaints. An oversized unit reaches the set temperature quickly and shuts off before it has run long enough to remove humidity, leaving the room cold and clammy. The short cycling also wastes energy — compressors draw a large surge at start-up — and shortens compressor life. A correctly sized unit runs for long stretches on a hot day, which is exactly what it should do.
Twelve thousand BTU per hour. The term is historical: it is roughly the rate of cooling you would get from melting one ton of ice over 24 hours, from the days when ice was the refrigeration technology. It has nothing to do with the weight of the equipment. In India and much of Asia units are sold in tons — 1 ton suits about a 120 to 150 sq ft bedroom, 1.5 ton about 180 to 200 sq ft.
Divide BTU per hour by 3,412. A 12,000 BTU/hr unit is about 3.5 kW of cooling capacity. Note that this is capacity, not electricity consumption — the whole point of a heat pump or air conditioner is that it moves several units of heat for each unit of electricity, so a 3.5 kW cooling unit might draw closer to 1 kW from the wall depending on its efficiency rating.
For a single room unit, a rule-of-thumb figure like this one is usually adequate to pick a size. For a whole-house system, ducted installation, or a heat pump, get a Manual J calculation. It accounts for actual wall construction, window specification and orientation, air infiltration, duct losses and local design temperatures, and it frequently comes out well below the square-foot estimate — which is a large part of why so much installed equipment is oversized.
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