Air conditioner size is measured in kilowatts, not the physical box on the wall, and getting it right is the difference between a system that quietly keeps up and one that costs more and never feels comfortable. Here is how to go from your room to a sensible kW range, what shifts that number, and how we size it against real systems.
What 'size' actually means
When we talk about the size of an air conditioner we mean its capacity in kilowatts (kW) of cooling or heating, not how big the unit looks on the wall. A small bedroom split might be 2.5kW; a large open-plan living area might need 7 to 8kW; a whole home on ducted is sized differently again.
As a rough starting point, cooling capacity scales with floor area, very roughly a couple of hundred watts per square metre for a normal room. That gets you a ballpark, but it is only ever a starting point, because two rooms of the same size can need quite different systems once you account for the things below.
A rough room-by-room guide
Treat these as a starting range for a typical, reasonably insulated Sydney room with standard ceilings, not a final answer:
- Bedroom or small study (up to ~20m²): around 2.5kW - Medium bedroom or small living room (20 to 30m²): around 3.5kW - Living or family room (30 to 45m²): around 5 to 6kW - Large open-plan living and kitchen (45 to 60m²+): around 7 to 8kW, sometimes a multi-split or ducted
If your room sits near the top of a range, or has any of the factors in the next section, it is usually safer to step up a size, but not by miles, because oversizing has its own problems.
Why the right size beats 'bigger is safer'
It is tempting to just buy big to be safe. It backfires. An oversized system cools the air quickly then switches off, short-cycling on and off without running long enough to pull humidity out, so the room feels cold and clammy and the compressor wears faster.
An undersized system has the opposite problem: it runs flat out on the hottest days, never quite gets there, and uses more energy doing it. The right size runs steadily, holds the temperature, dehumidifies properly and draws the least power. Size is the single biggest comfort decision you make, before brand, before features.
What actually moves the number
Floor area gets you in the ballpark; these push it up or down from there:
- Ceilings: anything above standard height is more air to condition. - Glass: large windows, and especially west-facing ones that cop the afternoon sun, add a lot of heat load. - Insulation: a well-insulated room holds temperature; an uninsulated top-floor room under a hot roof does not. - Orientation and shade: north and west rooms run hotter than shaded southerly ones. - The room's job: kitchens, and rooms full of people or electronics, carry more heat.
This is why a chart alone is never quite right, and why we ask about these when we size a system rather than just measuring the floor.
How we size it, against real systems
Most articles stop at a chart. Ours does not. Our capacity calculator asks a handful of quick questions, room size, ceilings, windows, orientation, applies the same rules we use when we quote, and gives you a sensible kW range for your space.
Then it does the part a chart cannot: it matches that range to real systems we actually install and shows each one's running cost from the official energy figures, so you are choosing between systems that genuinely fit, not guessing. It takes about thirty seconds.
From a kW range to the right system
Once you have your range, two things narrow it down. Efficiency: two systems of the same size can use noticeably different amounts of power, which you can line up on our compare page. And type: a single split for one room, a multi-split for a few, or ducted for a whole home.
If you would rather not work it out yourself, send us a few photos of the space and we will size it, recommend a system that suits, and give you a fixed price.