What size air conditioner do I need? Room size chart

What size air conditioner do I need for my room?

Measure the floor. Length times width, in metres. An open-plan kitchen, dining and lounge counts as one room if there is no door between them, because the air does not know where the rug ends.

Multiply by 120 watts. That is the middle of the 100 to 140 W/m² band most Australian installers work from, and it is the starting number our air conditioner size calculator uses. A 25 m² lounge comes out at 3.0 kW of cooling.

Then adjust for the four things that actually move the load:

• Sun. Big west-facing glass adds roughly 18 percent. A shaded, south-facing room takes about 10 percent off.
• Insulation. No ceiling batts or a leaky old house adds roughly 18 percent. A well-sealed, insulated room takes about 5 percent off.
• Ceiling height. A 2.7 metre ceiling adds about 10 percent. A raked or 3 metre-plus ceiling adds about 20 percent.
• Climate. Hot places (Queensland, the NT, Perth, inland NSW and SA) add about 10 percent. Melbourne, Hobart and Canberra take a few percent off for cooling, though heating matters more there.

Then pick the smallest standard size that covers the number. If the load sits within about 8 percent of the smaller size, take the smaller size. Inverters stretch. A little under on paper is fine; a lot over is how you buy a clammy room.

Air conditioner room size chart (m² to kW)

This is the calculator's logic laid out as a chart. Standard means average sun, average insulation, a 2.4 metre ceiling and a temperate climate like Sydney or Adelaide.

What size room will a 2.5 kW split system cool?

A 2.5 kW split cools a standard room of up to about 20 m². That is a bedroom, a study or a small lounge. Put the same unit in a west-facing room with big glass and it tops out nearer 18 m². Add poor insulation to that and call it 15 m².

It is the smallest common wall split and the cheapest to buy. A 2.5 kW unit on a straightforward back-to-back wall is usually $1,500 to $2,500 installed in 2026. It is also the unit people most often regret buying for a lounge, because on a 40 degree afternoon it runs flat out and still loses. Most brands also sell a 2.0 kW for small bedrooms, though the price saving over a 2.5 kW is usually small.

Is 2.5 kW enough for a bedroom? For a normal 10 to 16 m² bedroom, yes, and it will run quietly at low speed most of the night. That quiet is the point. See our quiet bedroom air conditioner guide if noise is the worry.

What size room will a 3.5 kW air conditioner cool?

A 3.5 kW split cools a standard room of up to about 29 m². In a sunny west-facing room, about 25 m². With west glass and poor insulation, about 21 m².

This is the most sensible size for a large main bedroom or a normal lounge in a brick veneer home. It usually costs only a few hundred dollars more than a 2.5 kW, often $1,700 to $2,700 installed on a simple wall, and it gives you headroom on the handful of days a year when it matters.

Is 3.5 kW enough for a living room? For a closed lounge under about 29 m², yes. For an open-plan kitchen, dining and living space, almost never. Measure the whole open area, not the bit with the couch.

What size room will a 5 kW, 7 kW or 8 kW split system cool?

A 5.0 kW split cools a standard room of up to about 42 m², or about 35 m² with big west glass. It is the default for a living and dining room.

A 7.1 kW split, the size most people mean when they search for a 7 kW split system, cools a standard room of up to about 59 m². With big west glass that drops to about 50 m², and with west glass and poor insulation to about 42 m². This is the open-plan unit. A 5 to 7 kW split is usually $2,100 to $3,900 installed in 2026 on a standard install, with a 7.1 kW on its own more like $2,600 to $3,900. Two storey pipe runs, double brick and new circuits add to that. For your wall and your switchboard, get up to three local quotes.

An 8 kW split covers up to about 67 m² in a standard room, or about 56 m² with west glass. Not every brand makes one; many jump from 7.1 kW to 8.0 or 8.5 kW and then to 9.4 kW. Past about 60 m² of open plan, one big wall unit starts losing to two smaller heads or a ducted system, because a single outlet cannot throw air evenly into the far corners. Price both before you buy. Our multi-head and ducted comparison covers the trade-offs.

One thing that trips up 7 kW buyers: the electrical side. The unit's installation manual sets the circuit breaker and cable size, and a licensed electrician decides whether your switchboard can carry it or whether it needs a dedicated circuit. Larger splits often do. Budget about $300 to $900 for a new circuit, and $1,000 to $3,500 on top if the switchboard itself needs upgrading.

What is 2.5 HP in kW? Converting horsepower and BTU

Australian units are sold in kilowatts of cooling. Imported listings and some older installers still talk in horsepower (HP) or BTU. The conversion from BTU is exact: 1 kW equals about 3,412 BTU per hour. The horsepower figure is a trade convention, not a physics one, and it varies a little between brands.

So a 2.5 HP aircon is a 7 kW class unit, and it suits an open-plan room of up to about 59 m² in a standard house. A 2 HP unit is a 5 kW class unit and suits up to about 42 m². If a listing only gives HP, ask for the rated cooling and heating capacity in kW. If they cannot give it, keep scrolling.

Is it better to oversize or undersize an air conditioner?

Slightly undersized beats badly oversized. Badly undersized is worse than both.

An oversized split hits the set temperature fast, throttles down to its minimum, and if the minimum is still too much for the room it cycles off and on. The air gets cold before the coil has run long enough to pull moisture out, so the room feels cool and clammy. You paid more for the unit, you often pay more to run it, and it is louder at the start of every cycle.

A modern inverter that is a touch small just runs longer at a steady speed. On the three hottest days of the year the room might sit at 26°C instead of 24°C. The other 360 days it runs in its most efficient range.

An undersized unit, a 2.5 kW in a 35 m² lounge, runs flat out for hours and never gets there. That is the one to avoid. The chart above exists so you land in the sensible middle.

The upsell usually sounds like this: "Go up a size, just to be safe." Ask what load they calculated and what they assumed about windows and insulation. If the answer is a shrug, you are paying for their safety margin. Get up to three licensed installers to size it for your actual rooms.

What size ducted air conditioning do I need for a 3 or 4 bedroom house?

Ducted is sized on the whole conditioned area, but not at the full 120 W/m² for every room. You never cool every zone at full load at once, and a zoned system is designed around that. As a rough guide for a single storey home with ceiling insulation:

Two things matter more than the headline kW. First, zoning, so you can run the living zone in the afternoon and the bedrooms at night instead of the whole house. Second, where the thermostat lives. A hallway sensor in a four bedroom house is how a 16 kW system ends up cooling the linen press. Our ducted air conditioning cost guide breaks the price down by home size, and the Victorian rebate page covers the discount if you are replacing gas ducted heating. A ducted quote should start with someone in your roof space, so ask for a local ducted quote rather than a phone estimate.

How to check an installer's sizing on a quote

• The quote names the rated cooling and heating capacity in kW, not just a model family.
• The installer asked about the room area, ceiling height, which way the windows face, and insulation. If they sized it from the driveway, they guessed.
• For open plan, they measured the whole connected space.
• For ducted, you get a zone plan and a return-air location, not just a number of outlets.
• If they recommend a size bigger than the chart, they can tell you why in one sentence.

Run your rooms through the size calculator before anyone visits, then compare. When two quotes disagree by one size, ask each installer to explain the other's number. The one who can is usually the one to hire. Once you know the size, the running cost calculator shows what it will cost per hour, and up to three licensed installers can confirm it on site.