What size air conditioner do I need?
It depends on far more than square footage, but there is a starting point.
For a typical home, count 30 to 40 BTU/h per square foot. A 200 sq ft room needs roughly 6,000-8,000 BTU/h, which is a 6,000 or 9,000 BTU commercial unit.
Sizing table by floor area
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| Floor area | BTU/h needed | Tons | Commercial unit |
|---|---|---|---|
| 100 sq ft | 3,000-4,000 | 0.3 | 5,000-6,000 BTU |
| 150 sq ft | 4,500-6,000 | 0.4 | 6,000 BTU |
| 200 sq ft | 6,000-8,000 | 0.6 | 6,000-9,000 BTU |
| 300 sq ft | 9,000-12,000 | 0.9 | 9,000-12,000 BTU |
| 400 sq ft | 12,000-16,000 | 1.2 | 12,000-18,000 BTU |
| 500 sq ft | 15,000-20,000 | 1.5 | 18,000 BTU |
| 700 sq ft | 21,000-28,000 | 2.0 | 24,000 BTU |
Why the range is so wide
Two 200 sq ft rooms can genuinely need different units. These are the factors that move the number:
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| Factor | Effect on capacity |
|---|---|
| Good insulation instead of poor | Up to 40 % less |
| Large unshaded west-facing window | Up to 20 % more |
| 10 ft ceiling instead of 8 ft | 25 % more (volume drives it) |
| Top floor under the roof | 15 to 30 % more |
| Four people instead of two | About 800 BTU/h more |
| Kitchen or appliances running | Can be significant, depending on wattage |
What the BTU number means
A BTU per hour measures the heat the unit removes from the room, not the electricity it uses. 12,000 BTU/h is one ton of refrigeration and 3.52 kW of cooling capacity. A 12,000 BTU unit does not draw 12,000 W: it draws far less.
Oversizing is also a mistake
With too much capacity, the room hits temperature quickly, the compressor stops, and restarts a few minutes later. In those short cycles the unit barely dehumidifies, so the room feels cold and clammy. It also uses more energy starting up and wears out sooner.
Refining the number
The table is for orientation. For an estimate that reflects your actual room (real ceiling height, insulation, glazing, orientation, occupancy), use the calculator: it breaks down where every watt comes from so you can argue with the assumptions.
Sources and methodology
Conversions use exact constants: 1 BTU/h = 0.29307107 W and 1 frigorie/h = 1 kcal/h = 1.163 W. The component sizing model (load per volume by insulation level, 120 W per person, solar gain by glazed area) is set out in full on the air conditioner size calculator. It is a guideline estimate: a professional heat-load calculation considers considerably more variables.
Frequently asked questions
What size AC do I need for a 200 square foot room?
Roughly 6,000 to 8,000 BTU/h in a typical home with an 8 ft ceiling. That is a 6,000 or 9,000 BTU unit depending on how exposed the room is.
How many BTU per square foot?
30 to 40 BTU/h per square foot as a reference. Less for well-insulated, shaded rooms; more for top floors or heavy west-facing glazing.
How many tons is 12,000 BTU?
Exactly one ton of refrigeration, which is also 3.52 kW of cooling capacity.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools fast, stops and restarts. It dehumidifies poorly, uses more energy in start-ups and lasts less.
Can I add rooms together?
Not directly. Each room is sized separately, and a system serving several must account for the fact that they rarely peak at once.