What size split system do I need? A room-by-room guide
By True Line Air Conditioning · 5 min read
Get the size right and a split system disappears into the background of your life: quiet, effective, cheap to run. Get it wrong in either direction and you pay for it every summer. Here’s how we actually size systems for Melbourne homes — the same logic behind our 30-second calculator.
The rule of thumb: about 0.14 kW per square metre
For a typical Melbourne home — standard ceilings, reasonable insulation — allow roughly 0.14 kW of cooling capacity per square metre of floor area. Then round up to the nearest size actually sold, never down. A system that has to work flat-out on a 38° afternoon is louder, thirstier and shorter-lived than one with capacity in reserve.
| Room | Approx. area | Cooling needed | Unit we’d fit |
|---|---|---|---|
| Small | ~12 m² | ~1.7 kW | 2.5 kW |
| Medium | ~20 m² | ~2.8 kW | 3.5 kW |
| Large | ~32 m² | ~4.5 kW | 5 kW |
| Open plan | ~50 m² | ~7.0 kW | 7.1 kW |
What changes the answer
- Ceiling height.Cathedral or 3-metre ceilings add air volume the rated area doesn’t capture — size up.
- West-facing glass.A wall of afternoon-sun windows can push a “medium” room into “large” territory.
- Insulation. Uninsulated period homes leak heat both ways; newer builds hold their temperature with less capacity.
- Open plan. If air flows freely between kitchen, dining and living, size for the whole connected space, not the corner the unit hangs in.
Why oversizing costs you twice
Bigger is not safer. A heavily oversized unit costs more to buy, then short-cycles — blasting the room cold, switching off, and repeating — which wears the compressor and dehumidifies poorly, so the room feels clammy at the “right” temperature. The goal is matched capacity, not maximum capacity. We’ll tell you when the smaller unit is the better unit; our prices are published either way, so there’s no incentive for us to upsell capacity you don’t need.
Cooling multiple rooms
We fit one single-head split system per room — each room gets its own properly-sized unit with its own controls. Bedrooms usually need only a 2.5 kW unit ($2,399 fully installed as an Essential package), so cooling three bedrooms costs less than most people guess. The calculator totals it per room in 30 seconds, and the final size is always confirmed with a free in-home measure before you commit to anything.
Larger spaces and businesses
For big open-plan areas, shops and offices we install single split systems up to 18 kW. Commercial sizing has extra variables — occupancy, equipment heat, door traffic — so those start with a free site consultation rather than a rule of thumb.
Know what your aircon will cost before you call.
Transparent published pricing, sized for your rooms in 30 seconds.