In Australia, heating and cooling a room mostly means a reverse-cycle split system, sold by capacity in kilowatts (kW). Too small and the room never gets comfortable on a 40°C afternoon or a frosty morning. Too big and the unit short-cycles, switching on and off without drying the air, and it costs more to buy. Getting the size roughly right matters more than buying the biggest unit you can afford.
Quick estimate
A practical starting point for cooling is about 0.15 kW per m² of floor area (150 W/m²) for a room with a 2.4 m ceiling. Then adjust:
- Add 10–20% for a west-facing or very sunny room, a poorly insulated ceiling or a lot of glass.
- Take some off for a shaded, well-insulated room.
- Scale up for ceilings higher than 2.4 m, in proportion to the height.
- Add more for kitchens and rooms with lots of people or equipment.
- Allow for the climate: tropical and inland areas need more cooling, while Tasmania, Canberra and alpine areas are usually sized on heating.
Then round up to the next standard size. Split systems commonly come in 2.0, 2.5, 3.5, 5.0, 6.0, 7.1, 8.0 and 9.0 kW. A 20 m² bedroom at 0.15 kW/m² needs about 3.0 kW, so a 3.5 kW unit. A 40 m² open-plan living area needs about 6 kW, so a 6.0 or 7.1 kW unit depending on sun and glass.
Cooling and heating capacity
Reverse-cycle units list both a cooling and a heating capacity, and the heating figure is usually a little higher. In cold climates, check the heating output at low outdoor temperatures, since it drops as it gets colder. The zoned Energy Rating label shows how a model performs in hot, average and cold climates, which is a better guide than the star rating alone.
Ducted and whole-house systems
For ducted systems and whole houses, a room-by-room heat load calculation is the professional standard. It uses the actual insulation, glazing, orientation and local design temperatures, and sizes the ducts and outlets as well as the unit. Rules of thumb routinely oversize whole-house systems. Installation needs a technician with an ARC refrigerant handling licence, and a licensed electrician for the power supply.
What affects the load
- Ceiling insulation and roof colour
- Window area, orientation, shading and glazing type
- Wall insulation and construction
- Air leakage around doors and windows
- Number of occupants and appliances
- The local climate
Placing the units
Mount the indoor unit where it can throw air down the length of the room, out of direct sun and not blowing straight onto beds or couches. Keep the outdoor unit close, shaded if possible and with clear airflow, and plan the pipe run and condensate drain before you quote.
Ventilation
Kitchens, bathrooms, toilets and laundries need exhaust fans that meet NCC 10.8.2: 40 L/s for a kitchen and 25 L/s for a bathroom, toilet or laundry, discharging outdoors. The exhaust fan calculator works it out.
The air conditioner size calculator gives a quick size for any room, with the adjustments above built in.
Measuring a room for heating and cooling
Measure the floor length and width, and the ceiling height. Note the number of external walls, the size and type of each window, and which way they face. Check the insulation where you can: the ceiling space, the walls where you can, and whether the floor is a slab or timber on stumps. Ask the client which rooms feel cold or hot now. That's often the best clue to where the heat goes.
Common mistakes
- Sizing by floor area alone, ignoring ceiling height, windows and insulation
- Oversizing "to be safe", which costs more and works worse
- Ignoring the room's use: kitchens and sunrooms behave very differently from bedrooms
- Choosing a unit by cooling capacity without checking its heating capacity in a cold climate
- Forgetting ventilation, which affects both comfort and heat loss
When to call a specialist
Rules of thumb are fine for single rooms and replacements. For whole-house systems, new builds, extensions with large glazed areas, and anything that needs a grant or certification, use a qualified air-conditioning designer or installer with a full heat load calculation. The cost of the calculation is small compared with the cost of the wrong equipment.
Common questions
Can one split system cool several rooms?
A wall-mounted split cools the room it's in, and struggles with rooms it can't blow air into. For several rooms, a ducted system or a multi-head split with an indoor unit in each room works much better.
Does better insulation change the size?
Yes, a lot. Insulating a ceiling, replacing single glazing or sealing draughts can cut a room's load by a third or more. When a job includes both insulation and new air conditioning, size the equipment for the insulated building, not the old one.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.