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What Size AC Do You Need? BTU Guide by Room Size
What Size AC Do You Need? BTU Guide by Room Size
Short answer: as a starting point, allow roughly 500–600 BTU per square metre. That puts a 9,000 BTU unit at rooms up to about 16 m², a 12,000 BTU unit at 16–26 m², an 18,000 BTU unit at 26–38 m², and a 24,000 BTU unit at 38–50 m². Then adjust upward for direct afternoon sun, high ceilings, large windows, kitchens and crowded rooms.
The sizing table
| Room size | Capacity | Typical room |
|---|---|---|
| Up to 16 m² | 9,000 BTU | Small bedroom, home office, single room |
| 16–26 m² | 12,000 BTU | Master bedroom, small living room |
| 26–38 m² | 18,000 BTU | Living room, open-plan salon |
| 38–50 m² | 24,000 BTU | Large salon, small shop or office floor |
| 50 m² and above | 30,000+ BTU, or two units | Commercial space, very large open areas |
Measure the room's floor area first: length × width in metres. That number is the base for everything below.
The adjustments that actually change the answer
Floor area alone will get you close, but a 20 m² bedroom on a shaded north wall and a 20 m² bedroom with a west-facing glass balcony are not the same cooling problem. Work through these and adjust:
- Direct afternoon sun: add 10–20%. West and south-facing rooms absorb heat all afternoon and are still releasing it at night. This is the single biggest adjustment on the list.
- Ceiling above 2.7 m: add 15–20%. The table assumes a standard ceiling. Older buildings with high ceilings hold a much larger volume of air.
- Large or single-glazed windows: add 10%. Glass is where most heat enters a room.
- Top floor or directly under a roof: add 10–15%. Roof heat load is substantial and often overlooked.
- Kitchen: add 3,000–4,000 BTU. An oven, a hob and a refrigerator all dump heat into the room while you are trying to cool it.
- Room regularly holds more than two people: add about 600 BTU per additional person. Relevant for a salon used for guests or an office.
- Well-shaded, well-insulated, north-facing: subtract 10%.
A worked example
A 22 m² master bedroom on the top floor, west-facing with a large balcony door.
- Base for 22 m²: 12,000 BTU
- West-facing with afternoon sun: +15% → 13,800
- Top floor under the roof: +10% → 15,200
- Large glass door: +10% → 16,700
The answer is an 18,000 BTU unit, not the 12,000 the floor area alone suggested. This is exactly the situation where people buy the smaller unit, run it constantly, and never get the room cold.
What goes wrong when the size is wrong
Undersized: the expensive mistake
An AC that is too small for the room never reaches the temperature you set. It runs at maximum output continuously, all day, and the room settles somewhere warmer than you wanted. That means the highest possible electricity consumption combined with the least comfort — and on an inverter unit it also destroys the efficiency advantage you paid extra for, because the compressor never gets to throttle down to its low-power state.
It is also hard on the machine. A compressor running flat out for months wears faster than one that spends most of its time cruising.
Oversized: the subtler mistake
An oversized unit cools the air quickly, hits the setpoint and switches off — then repeats. Two problems follow. First, on a non-inverter unit each restart is another current surge, which on a generator is exactly what you are trying to avoid. Second, an AC dehumidifies while it runs, and a unit that only runs in short bursts never runs long enough to pull moisture out of the air. You end up with a room that is cold and clammy rather than cold and dry.
You have also paid more than you needed to, both for the unit and for the electrical capacity to run it.
How BTU relates to amperes on your generator
BTU measures cooling output, not electricity consumed. Two 12,000 BTU units can have quite different current draws depending on their efficiency and whether they are inverter models.
If you are on a shared generator subscription, the number you care about is the rated current in amperes, listed on the specification of every unit in our range. Compare that directly against your allowance. As a rough guide, going from 9,000 to 12,000 BTU is not a small step in current draw — so getting the sizing right is also a way of not buying more electrical load than you need.
Note too that the capacity you choose interacts with the inverter decision. An oversized non-inverter unit is the worst combination for generator power: maximum surge, maximum frequency of restarts. See inverter vs non-inverter air conditioners for the full explanation, and which appliances are safe on a generator for how to read the current figures.
One big unit or two smaller ones?
For a single open space, one correctly sized unit is almost always better — it is cheaper to buy, cheaper to install and easier to run.
Two units make sense when the space is split by walls or doorways, because cold air will not travel around corners. A 24,000 BTU unit in a salon will not cool an adjoining bedroom through a doorway, no matter how powerful it is. Cooling capacity does not flow between rooms; it only reaches where the air reaches. If you have two rooms you actually use, budget for two units sized to each room rather than one large unit sized to the total area.
Before you buy: a short checklist
- Measure the room in metres and calculate the floor area
- Find the base capacity in the table above
- Apply the adjustments for sun, ceiling height, windows, floor level and use
- Round up to the next standard capacity, not down
- Check the rated current against your generator allowance
- Confirm there is a viable outdoor unit position with airflow
Browse capacities from 9,000 BTU upward in our split air conditioner range, or compare formats first in our guide to split ACs, portable ACs and air coolers. If you send us your room dimensions on WhatsApp, we will tell you the capacity you need.
Frequently asked questions
How many BTU do I need per square metre?
Roughly 500–600 BTU per square metre as a starting point, before adjusting for direct sun, ceiling height, window area and heat sources in the room.
What size room does a 12,000 BTU AC cool?
Approximately 16–26 m² under normal conditions. A room with strong afternoon sun, a high ceiling or large windows in that range may need 18,000 BTU instead.
Is 9,000 BTU enough for a bedroom?
For a bedroom up to about 16 m² without heavy sun exposure, yes. Larger bedrooms, top-floor rooms or west-facing rooms usually need 12,000 BTU.
What happens if my AC is too big for the room?
It cools quickly then switches off repeatedly, which means it never runs long enough to dehumidify properly. The room can feel cold and damp, and on a non-inverter unit the frequent restarts create repeated current surges.
Should I round the BTU up or down?
Round up. An undersized unit runs continuously without reaching the target temperature, which costs more to run and delivers less comfort than a slightly oversized one.
Can one AC cool two rooms?
Not effectively. Cold air does not travel around walls or through doorways in any useful volume. Two rooms you use regularly need two units sized individually.