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
The standard rule of thumb is about 20 BTU per square foot — roughly 600 BTU per square metre — for a room with normal 2.7 m ceilings. From there you adjust: very sunny rooms need about 10% more, shaded rooms 10% less, add roughly 600 BTU for each person beyond two, and kitchens need about 4,000 BTU extra for appliance heat. Enter your room above and the calculator does exactly this, then rounds up to the nearest standard unit size (9,000, 12,000, 18,000 BTU and so on).
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.
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.
How many BTU is 1 kW of cooling?+
1 kW of cooling equals about 3,412 BTU/h. So a 12,000 BTU unit delivers roughly 3.5 kW of cooling. Note that the electrical power drawn is much lower: a modern inverter unit with SEER around 7 uses roughly 0.5 kW of electricity to produce 3.5 kW of cooling.
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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