Most bedrooms need a 2.5kW air conditioner, most living rooms need 5 to 7kW, and big open plan areas need 7kW or more. To check your own room, allow about 150 watts of cooling for every square metre of floor, then adjust for ceiling height, insulation and sun.
That’s the short answer to “what size air conditioner do I need?”. The longer answer is worth ten minutes, because the wrong size costs you twice: once when you buy it and again on every power bill. It’s why installers such as Brisbane’s Current & Co measure the room before they quote a size. This guide covers the size chart, the formula, and the things a chart can’t see.
Key points
- Air conditioner size means cooling capacity, measured in kilowatts (kW).
- Floor area gets you close. Ceiling height, insulation, windows and climate decide the final number.
- Oversizing is not a safe bet. In a humid climate it leaves the room cold and clammy.
- If you land between two sizes, go up one size, not two.
How is air conditioner size measured?
Air conditioner size is the unit’s cooling capacity in kilowatts (kW), not its physical dimensions. A 5kW unit can remove about twice as much heat from a room as a 2.5kW unit.
Capacity is also different from the electricity the unit uses. A modern 2.5kW split system usually draws well under 1kW of power to deliver 2.5kW of cooling. That’s why the energy rating label shows capacity and energy use as separate figures.
Split systems are sold in set steps, most commonly 2.5kW, 3.5kW, 5kW, 6kW, 7.1kW and 8kW. Your job is to work out the capacity the room needs, then pick the nearest step.
Air conditioner room size chart
Use this air conditioner size chart as a starting point. It assumes a standard 2.4 metre ceiling, average insulation and a room with doors you can close.
Room size
Typical rooms
Air conditioner size
Up to 15 m²
Small bedroom, study, nursery
2 to 2.5kW
15 to 25 m²
Main bedroom, home office, small lounge
2.5 to 3.5kW
25 to 35 m²
Lounge room, bedroom with ensuite
3.5 to 5kW
35 to 50 m²
Large living room, kitchen and dining
5 to 7.1kW
50 to 60 m²
Open plan living area
7.1 to 9kW
Over 60 m²
Very large open plan space
9kW or more, two units, or ducted
Published charts don’t agree with each other. One manufacturer rates its 5kW unit for rooms of 25 to 34 square metres, while another rates the same capacity for 34 to 47. Treat any chart as a range to test against your own room.
How to calculate what size air conditioner you need
You can estimate the size yourself in three steps with a tape measure and a calculator.
Step 1: Measure the floor area
Multiply the room’s length by its width in metres. A lounge that is 6 metres by 5 metres is 30 square metres.
If the room opens onto a hallway, kitchen or dining area with no door, add that space too. The air conditioner will be cooling it whether you count it or not.
Step 2: Multiply by watts per square metre
The usual rule of thumb is 150 watts per square metre for a 2.4 metre ceiling. Taller rooms hold more air and need more capacity:
- 2.4 metre ceiling: 150 watts per square metre
- 2.7 metre ceiling: 160 watts per square metre
- 3 metre ceiling: 175 watts per square metre
Step 3: Convert to kilowatts and pick a size
Divide by 1,000 to get kilowatts. For the 30 square metre lounge with a standard ceiling, that’s 30 x 150 = 4,500 watts, or 4.5kW. The nearest size up is a 5kW split system.
Put the same lounge in an old Queenslander with 3 metre ceilings and the sum becomes 5.25kW. That pushes you to a 6kW unit.
Check it with an air conditioner size calculator
The rule of thumb errs on the generous side. For a second opinion, try the free FairAir cooling load calculator from the Australian Institute of Refrigeration, Air Conditioning and Heating. It asks about insulation, windows, shading and location.
The gap can be large. CHOICE ran an insulated 30 square metre room with shaded windows through it for Brisbane and got 2.6kW. Most commercial guides would recommend 3.5 to 5kW for the same room.
What else affects the size you need?
Floor area is one of four things that drive the result. Insulation, orientation and climate are the others, and each can move you up or down a size.
Insulation
Ceiling insulation makes the biggest difference. In CHOICE’s modelling, the same 30 square metre room needed 2.3kW with roof insulation and 2.9kW without it. That’s about a quarter more capacity just to fight heat coming through the ceiling.
Windows and orientation
Glass lets in far more heat than a wall does. A west facing room with big unshaded windows cops the afternoon sun right when the day is hottest. It needs more capacity than a shaded south facing room of the same size.
Blinds, awnings and window tinting all bring the number down.
Climate and humidity
Where you live changes the answer. CHOICE’s figures for that same room range from 1.8kW in Hobart to 2.6kW in Brisbane and 3kW in Perth.
In Queensland, humidity matters as much as temperature. The unit has to pull moisture out of the air as well as heat.
How the room is used
People, ovens, cooktops and big televisions all add heat. A kitchen and living area full of family at 6pm needs more cooling than a spare bedroom used at night.
What size air conditioner do I need for each room?
Bedrooms
Most bedrooms suit a 2.5kW split system. Step up to 3.5kW for a main bedroom over about 20 square metres, one with an open ensuite or walk-in robe, or one that faces west.
Bedrooms are mostly cooled at night, when the heat load is lower. There’s rarely a case for going bigger than 3.5kW.
Living rooms
A separate lounge of 25 to 35 square metres usually lands on 5kW. Go to 6kW or 7.1kW if the ceilings are high or one wall is mostly glass.
Open plan living areas
Open plan spaces are where sizing most often goes wrong. People measure the lounge and forget the kitchen and dining area attached to it. Measure the whole connected space. Most combined kitchen, dining and living areas need 7 to 9kW.
Past about 60 square metres, or in an L-shaped space, one wall unit struggles to push air into the far corners. Two smaller units or a ducted system will often do a better job than one very large one.
What happens if your air conditioner is the wrong size?
Both mistakes cost money, and bigger is not the safe choice.
If it’s too big
An oversized air conditioner cools the room fast, shuts off, then starts again a few minutes later. This short cycling means:
- the room swings between too cold and too warm
- the air stays humid, because the unit never runs long enough to dry it
- more wear on the compressor
- a higher purchase price for capacity you never use
Inverter models can turn themselves down, which helps, but only so far. In Brisbane’s humidity, a clammy room is the usual giveaway.
If it’s too small
An undersized air conditioner runs flat out and still can’t reach the set temperature on a hot day. Signs include:
- the unit never cycles off
- power bills that keep climbing
- hot spots at the far end of the room
If your room falls between two sizes, choose the next size up. CHOICE gives the same advice: a room that needs 6kW suits a 6 to 6.5kW unit. Jumping two sizes is where the trouble starts.
What size ducted air conditioner do I need?
Ducted air conditioning is sized on the rooms you’ll run at the same time, not the whole floor plan. Add up the area of those rooms and apply the same 150 watts per square metre. A home with 100 square metres of living space to cool at once needs about 15kW.
Zoning is what keeps this sensible. Cooling the living areas by day and the bedrooms by night lets a smaller system cover a bigger house.
Check your switchboard before you choose
Size affects the electrical work as well as the cooling. A split system is normally wired on its own dedicated circuit, and larger units draw more current. In older homes the switchboard may not have the space or the capacity for it, which can add an upgrade to the job.
Ask your installer to look at the switchboard when they measure the room. Install day is a bad time to find out.
Frequently asked questions
What size room will a 2.5kW air conditioner cool?
A 2.5kW air conditioner cools a room of about 15 to 17 square metres with a standard ceiling, or up to 20 square metres if the room is well insulated and shaded. That covers most bedrooms and studies.
What size room will a 3.5kW air conditioner cool?
A 3.5kW air conditioner suits rooms of roughly 20 to 25 square metres, such as a large main bedroom or a small lounge.
What size air conditioner do I need for a 30 square metre room?
By the rule of thumb, a 30 square metre room needs about 4.5kW, so a 5kW unit. A well insulated, shaded room that size can often run on 3.5kW.
How many kW per square metre do I need for air conditioning?
Allow 0.15kW (150 watts) per square metre for a 2.4 metre ceiling, 0.16kW for 2.7 metres and 0.175kW for 3 metres.
Is it better to oversize or undersize an air conditioner?
Neither. Aim for the right size, and if you’re between two, go one size up. A slightly larger inverter unit can turn itself down. An undersized one has nowhere to go on a 35 degree day.
Getting the size right the first time
A chart and a formula will get you within one size of the right answer. The last step is someone standing in the room, looking at the ceiling height, the glass and the afternoon sun.
If you’re in Brisbane, Current & Co installs Daikin and Mitsubishi Electric air conditioning. They assess the room onsite and give you an upfront price before any work starts, and as electricians they can look at the switchboard on the same visit.
Whoever you use, ask them to show you how they arrived at the size. A good installer will have a number and a reason.