Air Conditioner Sizing Calculator
Too big is worse than too small — here's the right kW.
Estimate the reverse-cycle split-system air conditioner size — in kilowatts — a room really needs, starting from its floor area and adjusting for the factors that matter: ceiling height, sun, insulation, climate zone and how many people use it. The answer snaps up to the common Australian split-system sizes. It's a rules-of-thumb estimate — for a big, glassy or open-plan room a licensed installer's heat-load calculation is the final word.
Estimates from your room size and typical AU rules-of-thumb — a licensed installer's heat-load calc is the final word for the exact spec.
How the recommended kW is worked out
Floor area → base load → adjustments → snap to a split size
The calculator starts with a base load: the room's floor area times a cooling density. A bedroom or study uses about 0.125 kW/m²; a living or open-plan area runs hotter — more glass, more people, more appliances — so it uses about 0.15 kW/m² (the usual spread is 0.10–0.15). It then multiplies that base by four factors — ceiling height × sun × insulation × climate — and adds roughly 0.1 kW per person past two, plus a little for heat-producing appliances. Finally it rounds up to the nearest common split-system size, because air conditioners only come in set capacities.
Worked example
A 20 m² bedroom at 0.125 kW/m² is a 2.5 kW base load; with standard ceilings, average sun and insulation and a temperate climate it stays about 2.5 kW — a 2.5 kW split. A 40 m² open-plan living at 0.15 kW/m² is a 6.0 kW load, so a 6.0 kW split suits it. Turn on the higher precision levels and the factors below start to move those numbers.
What changes the size — beyond floor area
Floor area sets the base, but four factors adjust it — and they stack (multiply together), so a hot, sunny, poorly-insulated room can need a size or two more than the floor area alone suggests.
- Ceiling height — a 2.4 m ceiling is standard. A 2.7 m ceiling adds about 10% and a 3.0 m or raked ceiling about 20%, because there's more air to cool.
- Sun — a north- or west-facing room with big unshaded windows adds about 15% for the summer afternoon sun; a well-shaded, south-facing room subtracts about 10%.
- Insulation — a poorly-insulated older home adds about 20%; a well-insulated new home subtracts about 10%.
- Climate — the hot, tropical north adds about 15%; the cool, temperate south subtracts about 10%.
- People & appliances — add roughly 0.1 kW per person past two, and a bit more for a kitchen or other heat-producing appliances in the space.
Because the effects multiply, it pays to be honest about aspect and insulation. A 20 m² west-facing bedroom, for instance, moves from 2.5 kW to about 2.9 kW — which rounds up to the next split size, 3.5 kW.
Don't oversize — and it heats as well
Why bigger isn't better
An oversized unit cools the air to temperature so fast that it switches off before it has removed the humidity — so the room feels cold but clammy. That short-cycling also swings the temperature about, wears the compressor faster and costs more to buy and run. An undersized unit is the opposite problem: it runs flat out and never quite reaches the setpoint on a heatwave day. The sweet spot is a unit that matches the calculated load and runs steadily, dehumidifying as it goes.
One unit heats and cools
A reverse-cycle split is an air conditioner and a heater in one. Its heating output is usually rated about 10–20% higher than its cooling capacity, so a unit sized on the cooling load normally heats the same room comfortably — one unit does both jobs. The exception is genuinely cold-winter zones — Canberra, Tasmania, the alpine areas — where the winter heating demand can exceed the summer cooling demand; there, size on the rated heating kW and check the unit's low-temperature performance.
When to get a proper calc
This tool gets most rooms to the right size, but for a precise spec — especially large, open-plan, heavily-glazed or unusually-shaped rooms — a licensed installer's heat-load calculation is the accurate answer. It accounts for the exact window sizes and orientations, the wall and roof construction and air leakage. A licensed installer is required for the install in any case, so use this number to sanity-check their quote and understand what's driving it.
❓ Frequently asked Frequently asked questions
What size air conditioner do I need?
It comes down mostly to floor area, then ceiling height, sun, insulation and climate. A rough starting point is about 0.125 kW per m² for a bedroom and 0.15 kW per m² for a living or open-plan area — so a 20 m² bedroom is around 2.5 kW and a 40 m² open-plan living about 6.0 kW. Adjust up for high ceilings, west-facing glass, poor insulation or a hot climate, then round up to the nearest common split size (2.0, 2.5, 3.5, 5.0, 6.0, 7.1, 8.0 or 9.0 kW).
How many kW per square metre?
A common rule of thumb is about 0.125 kW/m² for bedrooms and 0.15 kW/m² for living or open-plan areas, with the usual spread running 0.10–0.15 kW/m². Living areas sit higher because they have more glass, more people and appliances. It's only a starting point — ceiling height, sun, insulation and climate can move the real figure up or down by 30–40%.
Does a west-facing room need a bigger unit?
Usually, yes. West- and north-facing rooms with unshaded windows cop the summer afternoon sun exactly when you want them cool, adding roughly 15% to the load. A 20 m² bedroom moves from about 2.5 kW to 2.9 kW, which rounds up to a 3.5 kW unit. External shading — blinds, eaves or an awning — cuts this back and can save you a size.
Is it bad to buy one that's too big?
Yes. An oversized unit cools the room so quickly that it switches off before it removes the humidity, leaving the room cold but clammy. That short-cycling causes temperature swings, wears the compressor faster, and costs more to buy and run. Size to the calculated load rather than rounding up to be safe — an undersized unit is a problem too, but bigger is not automatically better.
Will one reverse-cycle unit heat and cool?
Yes. A reverse-cycle split is an air conditioner and a heater in one, and its heating output is usually rated about 10–20% higher than its cooling capacity, so a unit sized on the cooling load normally heats the same room comfortably. The exception is genuinely cold-winter areas — Canberra, Tasmania and the alpine zones — where heating demand can exceed cooling; there, size on the rated heating kW and check the low-temperature performance.
Is this calculator accurate enough to buy from?
It's a solid ballpark that gets most rooms to the right size and a good way to sanity-check a quote. For a precise spec — especially large, open-plan, heavily-glazed or unusual rooms — a licensed installer does a full heat-load calculation using the exact window sizes and orientations, wall and roof construction and air leakage. A licensed installer is also required for the electrical work, so treat this as a planning estimate, not a final specification.
Where these figures come from
There's no single official number for the size of air conditioner a room needs — it depends on the floor area, the aspect, the ceiling, the insulation and the climate. The figures here are 2026 rules-of-thumb drawn from consumer and installer sizing guidance for Australia; they are approximate and room-dependent, not a specification.
- Cooling density — about 0.125 kW/m² for bedrooms and 0.15 kW/m² for living/open-plan areas (spread 0.10–0.15), consistent with consumer sizing guidance such as CHOICE's air conditioner buying advice.
- Adjustment factors — ceiling 2.7 m ×1.10 and 3.0 m ×1.20; sun ×0.90 (shaded) to ×1.15 (north/west glass); insulation ×0.90 (good) to ×1.20 (poor); climate ×0.90 (cool south) to ×1.15 (tropical north); about +0.1 kW per person past two. These mirror the room-by-room sizing tools published by Solar Choice and Australian air-conditioning retailers.
- Common split sizes — 2.0, 2.5, 3.5, 5.0, 6.0, 7.1, 8.0 and 9.0 kW, the capacities most reverse-cycle wall-split ranges are sold in.
- Reverse-cycle heating — rated heating output is typically about 10–20% higher than the cooling capacity, so a unit sized on cooling usually heats the same room too, except in cold-winter zones.
Last checked: July 2026. These are indicative rules-of-thumb and vary by room, aspect, construction and climate. A licensed installer's heat-load calculation will differ and is the precise answer; a licensed installer is required to carry out the installation.
Select the question that matches where you are right now.
The headline number is the recommended split-system size in kilowatts — the estimated cooling load rounded up to the nearest capacity air conditioners are actually sold in. The breakdown shows the estimated load, the base load from floor area, the adjustments applied, and the reverse-cycle heating output.
Use it to shortlist units and to sanity-check an installer's recommendation. If a quote is a size or two above this, ask why — a big ceiling, west glass or poor insulation can justify it, but so can an installer playing it safe.
It's not a full heat-load calculation. It doesn't measure your exact windows and their orientation, your wall and roof construction, or air leakage — the things that matter most in large, glassy or open-plan rooms.
Air conditioners come in set capacities, so the tool snaps the estimated load up to the nearest common split size. If your load is only a whisker above a smaller size, a well-shaded room can sometimes take the smaller unit.
Five things move the size the most: floor area, ceiling height, sun, insulation and climate — plus the people and appliances in the room.
Floor area is the base. A living or open-plan area uses a higher density (~0.15 kW/m²) than a bedroom (~0.125) because it has more glass, more people and more appliances generating heat.
These multiply the base. A high ceiling, north/west glass, poor insulation and a tropical climate each add 10–20%, and they stack — while a shaded, well-insulated room in a cool climate needs less.
Each person past two adds roughly 0.1 kW, and a kitchen or other heat-producing appliances in the space add more — which is part of why open-plan living zones run hotter than bedrooms.
A few habits keep the estimate honest and the unit right-sized.
Enter the real floor area (length × width), and at the Standard and Detailed levels set the ceiling height, aspect, insulation and climate honestly — guessing "average" for a hot west-facing room will under-size the unit.
Resist buying a size bigger "just in case". An oversized unit short-cycles and leaves the room cold but clammy. Match the load, and add shading — blinds or eaves — to cut it rather than up-sizing.
For large, open-plan or heavily-glazed rooms, have a licensed installer do a full heat-load calculation. Use this estimate to compare against their number and understand what's driving it.
Sizing the air conditioner is usually one line in a bigger home-and-running-cost budget. Model the rest of it too.
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