What Size Air Conditioner Do I Need? The Simple Guide to Getting It Right
The math should be straightforward: find out how many watts you need per square metre and multiply that by the floor area of your room.
In reality, the answer is rarely that simple. Most commercial calculators recommend 3.5 to 5 kilowatts for a 30 square metre room, which is a typical range, yet you can’t just take that figure and scale it up or down to match your room size.
Finding the right air conditioner size for a room involves considering how much energy efficiency you’re aiming for, the climate you live in, how many people are staying in the room, and ceiling height. Despite how many factors we’ve already mentioned, we still haven’t covered the full scope!
Keep reading to learn more about air conditioner capacity, how to calculate the right size for your room, and how to avoid choosing an oversized or undersized unit for your AC installation.
Table of Contents
Air Conditioner Size Is Measured in kW
In the AC market, “size” refers to something different from the physical dimensions of an air conditioning unit. When someone asks about your AC’s size, they want to know how fast it can heat or cool your space.
The rate at which an AC delivers heating or cooling is expressed in kilowatts (kW). A 5kW rating implies that the AC can deliver heating or cooling at a rate of 5 kilowatts under standard test conditions, but that’s not its absolute fixed capacity. It may heat or cool a space at a slower or faster rate when in use by the consumer.
Your AC’s capacity determines how fast your room reaches your preferred temperature when you turn it on. Typically, the bigger a room, the larger the AC size needed and vice versa. As expected, larger units are pricier than smaller ones.
However, you shouldn’t consider only your room’s size or your budget when selecting an AC size. We’ll explain why below.
Common Misconceptions About AC Size
You may have heard that bigger isn’t always better.
This statement couldn’t be truer when it comes to air conditioners. Though a larger unit has more cooling capacity, this additional capacity doesn’t translate to more comfort or higher efficiency. An oversized air conditioner can be less effective than a unit of the right size.
Many assume room size alone dictates the capacity you should buy. The floor area, measured in square metres, is usually the starting point, but technicians also need to factor in other considerations, including wall material, insulation quality, window orientation, climate, and room usage. More on each soon!
Another common belief is that air conditioners of the same size cost roughly the same to run.
How much electricity an AC consumes also depends on its energy efficiency, not just its size. One 3.5 kW model may carry cooling ratings of 5.5 / 5.0 / 5.5 stars across hot, average, and cold zones, while another 3.5 kW model may be rated 3.5 / 3.0 / 3.5. Both ACs are the same size, but the less efficient unit will generally use more electricity to cool your room at the same pace.
Sizing calculations are complex. Ensure you contact a professional for a consultation before deciding on what size of air conditioner to buy.
What Size Air Conditioner Do I Need for My Room?
Room size isn’t the only factor that influences AC size, but it's the starting point for sizing calculations in most cases.
Calculating the size of a room involves multiplying the floor area by the ceiling height.
For the table below, we worked with a cooling capacity estimate of 0.10–0.15 kW per m², or square metre, of floor area. Later on we’ll explain how to manually estimate what AC size your room may need using your room’s full size.
| Floor Area | Starting Capacity Before Adjustments | Typical Room Type |
| 10–20 m² | 2.0–3.5 kW | Small bedroom, study, home office |
| 20–40 m² | 2.5–6.0 kW | Large bedroom with ensuite, small living area |
| 40–60 m² | 5.0–8.0 kW | Large living room, open-plan kitchen |
| 60+ m² | 8.0+ kW | Open-plan areas, large lounge rooms |
These figures serve as general guidelines. Please adjust estimates based on the room’s ceiling height, wall insulation, climate conditions, and room usage.
Factors That Affect Air Conditioner Sizing (Besides Room Size)
Beyond the floor area, several factors determine what size air conditioner you need for your room, such as these.
Floor area alone doesn’t tell the complete story when sizing an air conditioner. Your room’s total volume matters just as much because the system must cool every cubic metre of air, not just the floor space.
Rooms with higher ceilings contain more air volume and require more powerful systems to maintain comfortable temperatures than rooms with lower ceilings.
For example, if a 5 m x 6 m room has a floor area of 30 m² and a standard 2.4 m ceiling, it contains about 72 m³ of air. Raise the ceiling to 3 m, and the same room contains 90 m³ of air. That extra 18 m³ means the air conditioner has more air to cool, so the room may need a higher-capacity unit than the floor area alone suggests.
Insulation affects the rate of heat transfer from outdoor to indoor air. In a home with well-insulated ceilings and wall cavities, outdoor heat doesn’t enter rooms as fast. The slower pace of heat transfer reduces the cooling load, so the AC does not need to work as hard or run for as long.
A 30 m² room with roof insulation may need about 2.3 kW of cooling capacity, while the same room without roof insulation may need about 2.9 kW.
That difference may also increase running costs, as heat transferred outside can easily return to a poorly insulated room.
The type of material your walls are made of affects how fast heat is transferred into and from indoor air. Poorly-sealed timber walls, for instance, let heat in and out easily, which calls for running the AC at a lower speed when the outdoor temperature drops and increasing the speed once it’s hotter outside.
Thicker walls, such as brick walls, don’t let heat in or out as easily. On particularly hot days, brick walls may continue releasing heat into the room after sunset. The AC then continues working hard even after the outdoor temperature lowers.
Your location affects what size air conditioner you need. A home in a milder climate such as Canberra, Hobart, or Melbourne may require less cooling capacity than a home of the same size in Darwin, Cairns, Townsville, or another hot and humid part of northern Australia.
Where does your window face? West, East, North, or South? A room with West- or North-facing windows may get hotter during midday when the afternoon sun is at its peak, while one with East- or South-facing windows may heat up briefly when the morning sun rises but gradually cool as the day progresses.
Window glazing also impacts how fast a room heats up and cools down. Single-glazed windows transfer heat faster. A space with double-glazed or low-emissivity glass windows may do fine with an AC that’s up to 30% smaller than the standard size typically required.
The longer you spend in a room, the longer you’ll run the AC. Your installer will also look at whether you operate any heat-generating appliances in that room and how many people are present in it at a time, as these details can lower or raise the system’s cooling load.
Lighting also contributes to internal heat gain, especially incandescent bulbs. Ensure the calculations account for all extra heat-generating sources.
How to Calculate the Right Air Conditioner Size
Three methods, each with varying degrees of accuracy, exist for figuring out what size air conditioner you need.
Manual calculation method
- Measure the length and width of your room’s floor and multiply both numbers to calculate the floor area. The floor area is measured in square metres or m².
- Multiply the floor area by a capacity allowance of 0.15kW–0.175kW per square metre, depending on your ceiling height. Online calculators typically use 0.15kW for rooms with a ceiling height of 2.4m, 0.16kW for 2.7m ceilings, and 0.175kW for 3m.
Using online AC size calculators
Online calculators simplify the estimation process by factoring in multiple variables at once. You’ll get more accurate estimates if the calculator allows you to enter detailed information about your space.
Some installers provide simplistic analyses that recommend larger capacity than you need. Look for calculators that account for window size, shading, window coverings, insulation and local climate conditions.
You can try the FairAir cooling load calculator. It’s provided by the Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH). It doesn’t calculate AC sizes, but it uses Australian climate data to calculate a room’s cooling load. Based on the estimated cooling load, you can choose the right AC size for your room.
Working with professional installers
In Australia, a professional AC sizing recommendation should be based on a cooling load calculation, not only floor area.
Your AC contractor or designer may use Australian load-estimation methods based on the AIRAH DA09, which is the air conditioning load estimation manual from the AIRAH.
A proper professional calculation factors in:
- Floor area
- Ceiling height
- Insulation
- Wall and roof construction
- Window size and orientation
- Glazing
- Local climate
- Ventilation
- Number of occupants
- Appliances used in the room
- The amount of heat entering the space during the hottest time of the day.
For ducted systems, the contractor should also consider:
- Which rooms need cooling and when they need it.
- How the home is divided into zones
- How air will move through the ducts
- Where the ducts will be installed.
- How many outlets each area needs.
When you receive a quote, ask the contractor how they calculated the cooling load and which room conditions influenced the recommendation.
Why Should You Aim for the Right Air Conditioner Size?
Choosing an AC of the correct capacity affects both your comfort and running costs.
If the appliance you select is too small for your space, it’ll work harder to reach the target temperature. In some cases, an AC may never bring the temperature down to your desired level if the room is much bigger than what it was designed for, especially if you live in a part of Australia with a tropical climate.
As a result, the system consistently overworks itself but never gets to your desired temperature. Since it’s always operating at maximum capacity, it removes much more moisture from indoor air than is necessary, leading to excessive air drying. The unit may consume more electricity than it should, still fail to properly cool the room, and, eventually, wear out more quickly.
An oversized air conditioner, on the other hand, can cool the room too fast without dehumidifying the air in the process. This phenomenon, called short cycling, happens when the unit runs frequent short cycles to achieve the target temperature. Your room gets too hot or cold while the air remains damp or clammy. In the long run, you end up paying for more electricity.
Getting the Right AC Size Starts with an Accurate Quote
It’s easier to choose the right air conditioner size when a professional installer assesses the room properly. Before buying a unit, share details about your room size, ceiling height, insulation, window placement, suburb, cooling needs, and budget. With these details, your installer can recommend a system with enough capacity to cool the space without wasting energy.
At BG Electrical, we make that process straightforward. Tell us your location, the type of air conditioning system you’re considering, and any installation preferences you have in mind. We’ll review the details and provide a free quote tailored to your exact needs.