Air Conditioner Size Calculator
Work out what size air conditioner you need before you shop. Answer five questions about the room and the calculator returns a recommended capacity in watts (5,000W is a 5.0kW unit), then prices a supplied-and-installed split system to match.
Sizing starts at roughly 125 to 150 watts per square metre, then adjusts for the factors below.
Air conditioners cool a volume, not a floor. Tall ceilings mean more cubic metres to move.
Afternoon western sun through unshaded glass is the single biggest heat load in most Australian rooms.
Bigger is not safer. An oversized unit cools the air fast, hits the thermostat, then shuts down before it has pulled moisture out, so the room feels cold and clammy and the compressor short-cycles its way to an early failure. Match the capacity to the load.
Check the rated capacity at the Australian test condition, not the headline number on the box. Manufacturers publish capacity at 35°C outdoor for cooling, and output falls away in a 42°C heatwave, which is precisely when you need it.
- Indoor and outdoor unit$1,400
- Refrigeration labour$600
- Electrical, pipework, brackets$450
💰 Ways to save
- Shade the glass before you upsize the unit. External blinds, an awning or a well placed tree on the western side can strip 500 to 900 watts off the calculated load, which is often the difference between a 5.0kW and a 7.1kW unit and around $900 of purchase price plus a permanently lower running cost.
- Ask the installer to quote the model one size down as well as their recommendation. Reverse cycle splits step in fixed increments (2.5, 3.5, 5.0, 7.1, 8.0kW), so if your calculated load lands at 5,300W you are choosing between a 5.0kW unit that runs near full tilt on the five worst days of summer and a 7.1kW unit that is loafing for the other 360 days. The smaller unit is usually the better buy.
- Book the install in autumn or early spring. Refrigeration mechanics are flat out from November to February and quotes carry a peak-season premium, while the same job in April often comes in 10 to 15 per cent cheaper with a install date inside a fortnight rather than six weeks.
- If you are cooling two adjoining rooms, price a single larger split against a multi-head system. One well placed 7.1kW unit in an open plan area almost always beats two 3.5kW heads on both purchase price and running cost, provided the airflow actually reaches both spaces.
How we estimate this
## What size air conditioner do I need?
Pricing reviewed: June 2026.
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Pricing reviewed June 2026. Indicative Australian costs, not a quote.
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Understanding air conditioner sizes in Australia
What size air conditioner do I need?
The honest answer is that room size sets the starting point and four other factors move it. The rule of thumb used across the Australian trade is roughly 125 to 150 watts of cooling capacity per square metre of floor area for a standard 2.4m ceiling in a reasonably insulated home. A 20m² bedroom therefore lands around 2,500 to 3,000 watts, which is the 2.5kW to 3.5kW class. A 35m² living room lands near 4,400 to 5,300 watts, which is the 5.0kW class. A 55m² open plan kitchen and living zone pushes past 7,000 watts and into the 7.1kW or 8.0kW class. That is the number this aircon sizing calculator starts from before it applies the adjustments.
Why the rule of thumb alone gets it wrong
Air conditioners cool a volume of air, not a floor plan, so ceiling height matters as much as area. Going from a 2.4m ceiling to a 2.7m ceiling adds roughly 12 per cent more air to move, and a raked or cathedral ceiling can add far more while also letting the hottest air stratify above head height. Insulation is the next lever. A room with ceiling batts, wall insulation and double glazing might sit 500 watts below the rule of thumb, while a 1960s brick veneer room with a bare ceiling cavity and single glazed aluminium windows can sit 700 watts above it. Orientation is the one most people underestimate: unshaded west or north-west glass can add 800 watts or more of solar gain in the late afternoon, which is exactly when the outdoor unit is also working hardest against a 38°C ambient. Finally, climate zone shifts the whole curve, because Darwin and Cairns carry a latent humidity load that Hobart simply does not have.
What an air conditioner costs to buy and install in 2026
Once you know the capacity, the price follows fairly predictably. In 2026 a supplied and installed split system in a straightforward back-to-back position runs about $1,600 to $2,200 for a 2.5kW unit, $2,000 to $2,800 for a 3.5kW, $2,600 to $3,600 for a 5.0kW, and $3,400 to $4,800 for a 7.1kW to 8.0kW unit. Premium inverter brands with better part-load efficiency and quieter outdoor units sit at the top of each band, and budget brands sit below it, but the gap has narrowed as inverter technology has become standard. Multi-head systems serving two to four rooms typically land between $4,500 and $9,000 depending on head count and capacity.
The install variables that move the quote
Two identical units can differ by $1,500 on install alone. A back-to-back install, where the indoor head sits directly opposite the outdoor condenser on the other side of the same wall, is the cheapest scenario because the refrigerant line is under two metres. Every extra metre of pipe run adds material and labour, and once the pipework has to travel through a roof cavity, down a cavity wall or around a corner of the house, the cost climbs quickly. A condenser that has to go on a first floor wall bracket or a roof platform adds rigging and often a working-at-heights setup. Many older homes also need a new dedicated circuit run from the switchboard, which is a licensed electrician job in its own right and typically adds $400 to $600. Removing and disposing of an old unit is a modest extra but has to be done by someone licensed to recover the refrigerant.
Sizing traps that cost you twice
Oversizing is the most common and most expensive mistake. A unit with far more capacity than the room needs will satisfy the thermostat quickly, cut out, then restart a few minutes later. That short-cycling stops the coil running long enough to strip humidity out of the air, so a room can hit 23°C and still feel clammy, and the repeated compressor starts wear the machine out ahead of schedule. Undersizing has the opposite failure mode: the unit runs continuously at full load on hot days, never quite reaching setpoint, and burns more electricity than a right-sized unit doing the same job comfortably. Because capacity comes in fixed steps, aim to land just above your calculated load rather than two steps above it.
Compliance, refrigerant and who can do the work
In Australia any work that opens the refrigerant circuit requires an ARC refrigerant handling licence, and the electrical connection requires a licensed electrician. That is not a formality: incorrectly charged systems lose efficiency, and improperly disposed refrigerant carries real penalties. Ask for the licence number on the quote and a certificate of electrical compliance on completion. Energy performance is disclosed through the Zoned Energy Rating Label, which reports capacity and efficiency for hot, average and cold zones separately, so compare units on the zone that matches where you live rather than on the star count alone. Use the estimate above as your shopping bracket, then have two installers confirm the capacity on site before you commit.
Frequently asked questions
What size air conditioner do I need for my room?
Start at roughly 125 to 150 watts per square metre. A 20m² bedroom needs about 2.5kW, a 35m² living room about 5.0kW, and a 55m² open plan area about 7.1kW. Then adjust upward for high ceilings (add roughly 400W for 2.7m, 900W for raked), poor insulation (add about 700W) and western glass (add about 800W), and downward for shaded southern rooms or a cool climate.
Is it better to buy a bigger air conditioner than I need?
No. An oversized unit reaches the set temperature too quickly and shuts off before it has removed humidity, so the room feels cold and damp, and the constant stopping and starting shortens compressor life. Because capacities come in fixed steps of 2.5, 3.5, 5.0 and 7.1kW, choose the step just above your calculated load rather than jumping two sizes.
How much does a split system cost supplied and installed in 2026?
About $1,600 to $2,200 for 2.5kW, $2,000 to $2,800 for 3.5kW, $2,600 to $3,600 for 5.0kW and $3,400 to $4,800 for 7.1kW to 8.0kW in a straightforward back-to-back position. Long pipe runs, a new dedicated circuit (typically $400 to $600) and upper-storey or roof-mounted condensers push those figures higher.
Does ceiling height really change the air conditioner size?
Yes, because the unit cools a volume rather than a floor area. Lifting a ceiling from 2.4m to 2.7m adds around 12 per cent more air, and a raked or cathedral ceiling adds more again while allowing hot air to sit above head height. As a working allowance, add roughly 400 watts for a 2.7m to 3.0m ceiling and around 900 watts for anything raked or above 3.0m.
Does a west-facing room need a bigger unit?
Almost always. Unshaded west or north-west glass can add 800 watts or more of solar gain in the late afternoon, which is the same hour the outdoor unit is fighting peak ambient temperature. A 25m² room that suits a 3.5kW unit on a southern wall frequently needs 5.0kW on a western one, unless you shade the glass with an external blind or awning first.
Can I install a split system myself?
No. Any work that opens the refrigerant circuit requires an ARC refrigerant handling licence and the electrical connection requires a licensed electrician. Ask for the licence number on the quote and a certificate of electrical compliance when the job is finished. DIY installs also void nearly every manufacturer warranty in the Australian market.
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