Short answer
Room size is only the starting point. Correct sizing depends on floor area, ceiling height, which way the room faces, how much glazing it has, insulation, and how the room is used. In Brisbane, orientation matters more than most people expect: a west-facing living room with large windows can need substantially more capacity than a shaded bedroom of identical floor area. Both undersizing and oversizing cause real problems, which is why capacity is calculated from the room rather than a chart.
What actually drives cooling load
A proper assessment accounts for all of these:
- Floor area and ceiling height, since volume is what has to be cooled
- Orientation, with west and north-west facing rooms taking the heaviest afternoon load
- Glazing: how much, which way it faces, and whether it is shaded or treated
- Insulation in the ceiling and walls
- Whether the space is open-plan or closed off from adjoining rooms
- How many people typically use it, and heat-producing appliances such as a kitchen
Why undersizing is a problem
An undersized system runs continuously at full output and still fails to reach the set temperature on the hottest days, which is exactly when you need it.
Running flat out constantly uses more power than a correctly sized unit cycling normally, so the cheaper purchase costs more to run and wears faster.
Why oversizing is also a problem
This one surprises people. An oversized system cools the air quickly, satisfies the thermostat and shuts down before it has removed much moisture from the room.
In Brisbane's humidity that produces a room that is cold but clammy, along with short-cycling that is hard on the compressor. Bigger is not safer.
The rough guide, and its limits
General rules of thumb based on floor area exist and can be a starting point for a conversation, but they ignore orientation, glazing and ceiling height entirely.
For a west-facing Brisbane living room with large windows, a floor-area rule of thumb will commonly undersize the system. That is precisely the room where it matters most.
Rather than quoting a number here that could be wrong for your room, the honest answer is that capacity should be calculated from your actual space.
What to have ready when you ask
Being able to describe these makes the conversation much faster:
- Approximate room dimensions and ceiling height
- Which direction the main windows face
- How much glazing there is, and whether it gets afternoon sun
- Whether the room is open to other spaces
- Whether the ceiling is insulated, if you know
General guidance. Correct sizing requires an assessment of the actual room and building.