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What size air conditioner do I need?

Choosing the right size can save you money and reduce your energy use.

man looking at size of air conditioner on wall
Last updated: 19 October 2021

Need to know

  • Room size is important, but it's not the only important factor
  • Insulation, location and orientation also have significant impact
  • Choosing the right size is vital to maximise performance and minimise running costs

No matter how hot under the collar you get, buying an air conditioner should never be an impulse purchase. It's an expensive, long-term commitment that has big implications for your power bill and comfort levels alike – so it's important to do your homework before installing one. 

While it's tempting to just opt for the biggest air conditioner you can get, bigger isn't always better. So how can you find out the capacity you need? We'll walk you through it.

What size air conditioner should you get?

CHOICE air conditioning expert Chris Barnes explains the factors that affect what size air conditioning unit you need.

The big four

While room size is an important consideration, every home is different and there are many other factors that will affect your air conditioning capacity requirements.

I saved hundreds on my last air conditioner purchase by calculating that a smaller capacity model would suit my home.

"Take some time to assess the room and figure out which size air conditioner you need," says CHOICE's air con expert Chris Barnes.

"I saved hundreds on my last air conditioner purchase by calculating that a smaller capacity model would suit my home."

These four things have the biggest impact on what size air conditioning unit you'll need:

  1. Room size: What's the length, width and ceiling height? While floor space is important, so is total volume – a room with high ceilings will require more energy to cool.
  2. Insulation: Are the ceilings and walls insulated? What's underneath and on top of the room? Ceiling insulation is one of the biggest factors in making your home thermally efficient.
  3. Location: Where do you live? A room in Darwin will need a more powerful air conditioner for cooling, compared to an otherwise identical room in Hobart. 
  4. Orientation: Which way does the room face? A large north- or west-facing window can let in a lot of heat in summer, whereas a shaded, southern-facing window will be cooler. 

CHOICE tip: Ceiling insulation has the biggest impact on how big an air conditioning unit you'll need. If you don't have roof insulation, consider installing it – it'll save you money in the long run, as you can get away with a smaller, cheaper air conditioner, as well as ongoing running costs.

Room size

Here's our rough guide to the air conditioner capacity (size) you'll need to cool a particular room size. 

Air conditioning capacity requirements per room size

Room size




Approx. capacity 


Price guide


Small (up to 20m2)


Bedroom, study, small kitchen






Medium (20–40m2)


Bedroom with ensuite, small lounge






Large (40–60m2)


Large bedroom, mid-sized lounge, large kitchen






Extra large (60+m2)


Open-plan areas, large lounges





Size matters

This is a ballpark guide, but we recommend that you do an accurate calculation so you don't end up with a system that's drastically over- or under-sized. Don't be tempted to go smaller to save money, or larger to keep your house feeling like a fridge. Remember, too, that the size of the unit could affect the cost of installation and replacement.

"Bigger isn't always better, and smaller isn't always more economical," says CHOICE's air con expert Chris Barnes. "Aim for the sweet spot."

Here's what can happen if you super-size or skimp on your air con:

  • Too big: The unit may run frequent short cycles to achieve the target temperature. This can mean:
    • the room gets too hot or cold
    • the unit doesn't dehumidify the air enough (so the room feels less comfortable)
    • power use increases
    • running costs increase
    • more wear and tear on the system.
  • Too small: The unit may have to run at maximum output more often. This can mean:
    • the unit dries out the air too much
    • more wear and tear on the system
    • power use increases.

CHOICE tip: Choose a model with capacity that's either just right, or slightly more than you'll need for the room. For example, if the room needs a 6kW model, then go for a unit with a rated cooling capacity in the range of 6–6.5kW.


Some installers and online calculators offer only a simplistic analysis and may tend to recommend a larger capacity than you really need.

Find a calculator that takes all of the room's details into account, including window size, shading, window coverings, insulation, local climate, etc. The more information you can include, the more accurate the calculation will be.

The cooling load and heating load calculators on, by the Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH), are worth using; they allow you to factor in a lot of details about the room, its insulation, window orientation and more, to get an accurate guide to the right air-conditioner size. also has a size calculator you might try. Alternatively try the calculators on manufacturer and installer websites, but we think these tend to overestimate the capacity you need.

Our air conditioner buying guide has lots of info about buying the best air con for your needs, and you can easily compare the cooling capacities of different models in our air conditioner reviews.

We care about accuracy. See something that's not quite right in this article? Let us know or read more about fact-checking at CHOICE.