Home & DIY
What size air conditioner do you need (BTU)?
BTU and kW needed to cool a room based on size, sun exposure and occupants — with the right split/window unit size.
Quick answer
Air conditioner sizing is one of the few purchases where bigger is genuinely worse. An oversized unit cools the air fast but shuts off before it has time to dehumidify, leaving the room cold and clammy, and the constant on-off cycling wears out the compressor. An undersized one runs flat out all day without ever reaching the set temperature. The sweet spot is a unit sized to run in long, steady cycles.
Cooling capacity needed
12,000 BTU/h
- In kilowatts
- 3.5 kW
- Recommended unit size
- 12,000 BTU
Running it all summer? AC electricity cost calculator
Rule of thumb of ~600 BTU per m² (~56 BTU per sq ft) for standard 2.7 m ceilings, adjusted for sun, occupants and kitchen heat. Poor insulation or high ceilings can add 10-20%. For multi-room or whole-house systems get a professional sizing.
How it works
Rule of thumb: about 600 BTU per m² (≈ 56 BTU/sq ft) for normal 2.7 m ceilings. Then the calculator applies four corrections and rounds up to the next size actually sold — 7k, 9k, 12k, 18k, 24k, 30k, 36k — because there is nothing in between on the shelf.
| Condition | Correction |
|---|---|
| Strong sun, or directly under an uninsulated roof | +10% |
| Shaded or north-facing | −10% |
| Each person beyond the first two | +600 BTU |
| The room is a kitchen | +4,000 BTU |
| Room size | Calculated BTU | Unit to buy | kW |
|---|---|---|---|
| up to 11 m² | up to 7,000 | 7,000 | 2.1 |
| 12–15 m² | 7,200–9,000 | 9,000 | 2.6 |
| 16–20 m² | 9,600–12,000 | 12,000 | 3.5 |
| 21–30 m² | 12,600–18,000 | 18,000 | 5.3 |
| 31–40 m² | 18,600–24,000 | 24,000 | 7.0 |
| 41–50 m² | 24,600–30,000 | 30,000 | 8.8 |
Read that table as the plain case — average exposure, two people, not a kitchen. Each of those adjustments can push a room into the band above: a 20 m² living room with large west-facing windows calculates at 13,200 BTU, which is no longer a 12,000 unit but an 18,000. It is worth checking where your room falls relative to the boundary, because a room sitting just under one is the case where the sun or an extra person changes the answer.
Prefer a fixed split if you can install outdoors — 2–3× more efficient and quieter than a portable. Portables: buy one size up from the calc, because the exhaust hose leaks heat back. Shop inverter split air conditioner after you know the BTU, not before.
The BTU figure is the easy part; where the quote grows is the installation. The condensate has to run away by gravity, so an indoor unit on a wall far from the outside needs a small pump — an extra cost, and the commonest source of the gurgling noise people complain about a year later. Two more things to settle before the installer arrives: the outdoor unit loses efficiency baking in full sun or boxed into a closed niche, and beyond about five metres of pipe run the system needs extra refrigerant added, which is not always included in the advertised price.
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Frequently asked questions
How many BTU do I need for a typical bedroom?+
A 12-15 m² (130-160 sq ft) bedroom with average exposure needs around 7,000-9,000 BTU, so the smallest standard split (9,000 BTU) covers almost every bedroom. Go to 12,000 BTU only if the room is over 20 m², gets strong afternoon sun, or is under an uninsulated roof.
BTU per m² — quick reference?+
About 600 BTU/m² for a normal room — multiply floor area by 600, then apply the sun, people and kitchen adjustments in the calculator. In imperial units that is roughly **56 BTU/sq ft**. Careful with the American charts you will find alongside it: those quote about **20 BTU/sq ft**, which is a genuinely different figure — around 215 BTU/m², a third of the European one. The two are not the same rule in different units. The gap comes from what is being sized: US charts assume an insulated envelope and a room-only cooling load, while the European figure reflects masonry walls, weaker glazing, stronger summer sun, and a unit that usually has to heat the same room in winter.
What happens if the air conditioner is too big?+
It short-cycles: it reaches the target temperature quickly, shuts off, and turns back on minutes later. This leaves humidity in the air (the room feels cold but sticky), consumes more energy than steady operation, and stresses the compressor. Comfort actually gets worse, not better.
Portable AC or split system — which one?+
A fixed split is 2-3× more efficient and much quieter — always prefer it if you own the place and can install the outdoor unit. Portables make sense for renters and occasional use: expect real-world performance well below the rated BTU, since the exhaust hose leaks heat back into the room. If you go portable, buy one size up from what the calculator suggests.
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