Getting the size right is the single most important decision when buying an air conditioner. Here's the simple rule we use — and a chart to match your room to a capacity.
Guide | Updated July 2026 | Climate Supply Co.
Buy an air conditioner that's too small and it'll run flat out on hot days, never quite cool the room, cost a fortune to run and wear out early. Buy one that's too big and it "short-cycles" — blasting cold then switching off before it's pulled the humidity out of the air, which is both wasteful and leaves the room feeling clammy. In South East Queensland's humid summers, sizing matters even more.
The good news: you can get very close with one simple calculation.
Floor area (m²) × 130 watts = capacity you need
then divide by 1,000 to get kilowatts (kW)
Worked example: a living room that's 6m long and 5m wide is 30 m². Multiply by 130 watts — that's 3,900 watts, or 3.9 kW. So you'd choose a unit around 4 kW (the nearest common sizes are 3.5 kW and 4.0 kW).
Measure the length and width of the room in metres and multiply them together. For open-plan areas — say a kitchen, living and dining that all flow together — add up the floor area of every zone the air conditioner will need to cool, not just the room it's mounted in.
Based on the 130 watts per m² rule, here's a starting point for typical rooms:
| Room size | Capacity needed | Typical room |
|---|---|---|
| Up to 20 m² | ~2.5 kW | Bedroom, study, nursery |
| 20 – 30 m² | 2.5 – 3.5 kW | Large bedroom, small living room |
| 30 – 40 m² | 4.0 – 5.0 kW | Living room, lounge |
| 40 – 50 m² | 5.0 – 6.5 kW | Open-plan living/kitchen |
| 50 – 65 m² | 7.0 – 8.5 kW | Large open-plan area |
| 65 m² + | 8.5 kW + or ducted | Whole zones — consider ducted |
The 130 W rule is a solid baseline, but a few things mean you should round up (or go one size larger):
Well-insulated, shaded or south-facing rooms can often go with the lower end of the range.
A note on accuracy: this rule gets you the right ballpark for a domestic room. When we install a system, we confirm the sizing properly for your home so you don't end up over- or under-sized. If you're not sure, send us the room dimensions and we'll tell you the right unit.
That's 20 m². At 130 W per m² it works out to 2,600 W, or 2.6 kW — so a 2.5 kW split system is the usual pick.
A little larger beats too small — but don't go way over. An oversized unit cools fast then shuts off before removing humidity, so the room feels cold and clammy and you waste energy. Aim for the right size, rounding up to the nearest available capacity.
That's ducted territory rather than a single split. You can still use the 130 W rule on your total floor area as a guide, but a ducted system should be sized with a proper heat-load calculation. See our split vs ducted guide.
Browse split systems by capacity, or tell us your room and we'll match it.
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