As a rule of thumb, a standard Australian room needs about 125 to 150 watts of cooling for every square metre of floor space. That puts a small bedroom at roughly 2 to 2.5 kW, a medium bedroom or small lounge at 2.5 to 5 kW, and an open-plan living area at 5 to 9 kW. At QuickAir, we’ve sized and installed heating and cooling for more than 20,000 Australian homes, and the size of the unit is the single decision that most affects how well it works and what it costs to run.
Size matters more than most people expect. A unit that’s too small will run flat out and still never quite cool the room. A unit that’s too big cools the air so fast that it switches off before it has pulled the humidity out, which leaves the room cold and clammy and wears the unit out with constant stopping and starting. The goal is to match the unit to the room, then adjust for the things that make your room different from the average one.
Below is the full method: a size table you can read off directly, a quick calculation you can do yourself, and the factors that push the number up or down.
What size air conditioner do I need? The short answer
To size an air conditioner for a room, do three things:
- Measure the floor area. Multiply the length by the width in metres. A room that’s 4 m by 5 m is 20 square metres.
- Apply a baseline. Allow roughly 125 to 150 watts per square metre for a standard room with average ceilings and insulation. So a 20 square metre room needs about 2.5 to 3 kW.
- Adjust for your room. Add capacity for tall ceilings, lots of sun, poor insulation, extra people, or a kitchen. Take some off for a well-shaded, well-insulated room.
That gives you a target in kilowatts, which is how air conditioners are sold in Australia. Match it to the closest unit, and when the number falls between two sizes, size up rather than down.
Air conditioner size chart by room size
The table below is a quick reference for how much cooling capacity a room typically needs, based on its floor area. It’s adapted from sizing guidance published by the consumer group CHOICE and matches what we see across the homes we work in.
| Room area (approx.) | Example room type | Typical cooling capacity |
| Up to 20 m² | Small bedroom, study, small kitchen | 2 to 2.5 kW |
| 20 to 40 m² | Large bedroom, small lounge, ensuite | 2.5 to 5 kW |
| 40 to 60 m² | Mid-size lounge, large bedroom | 4 to 6 kW |
| Over 60 m² | Open-plan living, large lounge | 5 to 9 kW |
Use this as a starting point, not a final answer. As a common reference point, a 5 kW split system suits a room of about 20 to 35 square metres. If your room sits near the edge of a band, or has any of the features in the factors section below, work through the adjustments before you settle on a size.
How to calculate air conditioner size yourself
If you want to work it out rather than read it off a chart, here’s the method step by step.
Step 1: Measure the room
Measure the length and width of the room in metres and multiply them for the floor area. Ignore doorways and small nooks. For open-plan spaces, measure the whole connected area that the unit will be expected to cool, not just the section near the wall.
A few things are worth noting while you’re measuring. A room on the top floor or under an uninsulated roof takes on more heat than the same room downstairs. A room with two or three external walls loses and gains more heat than one tucked in the middle of the house. And a room that opens onto a hallway or another space is effectively larger than its own four walls suggest. None of these change the floor-area figure, but they all point toward sizing up rather than down.
Step 2: Apply a baseline figure
There are two common baselines, and it helps to understand both.
- Metric method (kilowatts): Multiply the floor area by about 150 watts. A 24 square metre lounge needs roughly 3.6 kW, so you’d look at a 3.5 kW or 5 kW unit.
- BTU method (imperial): The U.S. Department of Energy suggests about 20 BTU per square foot of living space. To convert to kilowatts, divide the BTU figure by 3,412, since 1 kW is about 3,412 BTU per hour.
One thing worth knowing: the basic BTU-per-square-foot rule is a US guideline for cooling only, and it tends to come out low for Australian homes. A 180 square foot bedroom (about 17 square metres) works out to roughly 4,500 BTU, or about 1.3 kW, on that rule. Our own table puts a room that size closer to 2 to 2.5 kW. The difference is deliberate. Most Australian systems are reverse cycle, so they heat in winter as well as cool in summer, and our summers run hotter than much of the US. That’s why local sizing, and the kW table above, sit higher than the plain imperial rule. It’s also why we treat any rule of thumb as a starting point rather than a final number.
Step 3: Round to a real unit size
Air conditioners come in set sizes, commonly 2.5 kW, 3.5 kW, 5 kW, 6 kW, 7 kW and up. Match your figure to the nearest size, and when you’re between two, choose the larger one. A slightly oversized unit is easier to live with than one that can’t keep up on the hottest day of the year.
What changes the size of air conditioner you need
Floor area is only the starting point. Two rooms of the same size can need very different units. These are the factors that move the number, and the rough adjustments we apply.
| Factor | What it does | Rough adjustment |
| Ceiling height | Taller ceilings mean more air to heat and cool | Add capacity above 2.7 m |
| Insulation | Poorly insulated walls, roof or floors let heat in | Add for poor insulation |
| Sun and windows | Large west or north-facing windows gain heat | Add about 10% for a very sunny room |
| Shading | A well-shaded room needs less | Take off about 10% if heavily shaded |
| People | Each body adds heat | Add about 600 BTU for every person beyond two |
| Kitchen and appliances | Ovens, fridges and electronics add heat | Add about 4,000 BTU for a kitchen |
| Climate | Hotter, more humid regions need more | Varies by city |
Ceiling height is the factor people forget most often. Sizing tables assume a standard ceiling of about 2.4 to 2.7 metres. Raise that to a 3 metre ceiling or a raked roofline and you’ve added a large volume of air the unit has to condition, even though the floor area on your tape measure hasn’t changed. In rooms like that, the floor-area figure understates what you need.
Climate makes a real difference too. CHOICE notes that a 30 square metre room might need about 2.9 kW in Darwin but only around 2.3 kW in Sydney, purely because of the temperature and humidity difference. The Victorian Government’s energy advice makes the same point for reverse cycle systems: the right size depends on where you live, not just the room.
A worked example with adjustments
Say your basic calculation lands on about 10,000 BTU, or roughly 2.9 kW, for a 30 square metre lounge. Now factor in the details. The room faces west and gets strong afternoon sun, so add about 10%. It regularly holds three people, so add about 600 BTU for the extra person beyond two.
That takes you to roughly 11,700 BTU, or about 3.4 kW. On the chart alone you might have reached for a 3.5 kW unit, and here the adjustments confirm it. In other rooms the adjustments tip you up into the next size, which is exactly why they matter.
What happens if the air conditioner is the wrong size
Getting the size wrong costs you either way, in comfort and in running costs. Here’s the difference.
| Too small | Too big | Right size | |
| Reaching temperature | Struggles, never quite gets there | Cools very fast | Reaches and holds temperature |
| Humidity | Runs constantly, some dehumidifying | Switches off before removing humidity, leaves the room clammy | Removes humidity properly |
| Running cost | High, always working hard | High, constant stopping and starting | Efficient |
| Lifespan | Worn by constant running | Worn by short cycling | Longest |
An oversized unit is a more common and more expensive mistake than most people realise. It looks like the safe choice, but the short cycling wastes energy, leaves the air damp, and shortens the life of the unit. Whichever way you go, a well-matched unit reaches temperature, holds it, and does it using less power, which is also why we recommend regular servicing to keep it performing at the size it was chosen for.
When to get a professional size assessment
Rules of thumb and tables get you close, and for a single standard bedroom they’re usually close enough. For anything larger or more complex, the accurate answer comes from a proper load calculation that accounts for ceiling height, insulation, glazing, orientation and climate all at once. You can run one yourself through a detailed load calculator built by the air conditioning industry body AIRAH, or have a technician assess the room in person.
An on-site assessment is the most reliable option, because the right system depends on your actual home, not a rough guess. When one of our technicians assesses a job, they work through the type, size and layout of the space, the available spots for the indoor and outdoor units, the insulation and sun the room gets, how many rooms need cooling, your usage patterns, and any noise or air quality concerns. For a whole home or a large open-plan area, that assessment often points toward a ducted reverse cycle system rather than a single large split, because one oversized head unit rarely cools an open plan evenly.
Frequently asked questions
What size air conditioner do I need for a 20 square metre bedroom?
A 20 square metre bedroom typically needs about 2 to 2.5 kW. Add a little if the room has tall ceilings, poor insulation, or large west-facing windows.
What size air conditioner for a 12 by 12 room?
A 12 by 12 foot room is about 13 square metres, which usually calls for a 2 to 2.5 kW unit. A standard 2.5 kW split system is a common fit for a room this size.
How many kW do I need to cool a 50 square metre room?
A 50 square metre space generally needs around 5 to 6 kW. At that size, and especially in an open plan, it’s worth getting a load calculation rather than relying on the chart alone.
How many BTU is 5 kW?
5 kW is about 17,000 BTU per hour, since 1 kW equals roughly 3,412 BTU. A 5 kW unit suits a room of about 20 to 35 square metres.
What size air conditioner do I need for a living room?
Most living rooms fall between 4 and 7 kW, depending on floor area, ceiling height and how much sun they get. Open-plan living and kitchen areas often sit at the top of that range or above, and are the rooms most worth getting a proper assessment for.
Does a bigger air conditioner cool a room faster?
A bigger unit does cool faster, and that speed is exactly what causes trouble. It reaches temperature and switches off before it has removed the humidity, then starts again a few minutes later. That short cycling leaves the room damp, uses more power, and wears the unit out sooner.
Is it better to oversize or undersize an air conditioner?
Neither. A unit that’s a touch large is more forgiving than one that’s too small, but a badly oversized unit short cycles, wastes power, and leaves the room humid. Match the size to the room.
Get the size confirmed before you buy
Working out the size yourself is a smart first step, and the table above will get you close for most rooms. Before you spend on a new system, it’s worth having the number confirmed for your actual home, so you don’t pay for capacity you don’t need or end up with a unit that can’t keep up. You can book an on-site installation quote for a fixed $149, credited back to you if you go ahead with the install. Our technicians work across every capital city and give you a fixed written quote with no pressure either way. Get in touch or call us on 1300 730 896.
