Aircon size calculator

Most people size an air conditioner by guessing, and most people guess wrong. Too small and the unit runs flat out all summer without ever quite cooling the room. Too big and you've paid for capacity you don't need, while the unit short-cycles and leaves the air clammy.

Room size is the starting point, but it's not the whole story. A north-facing lounge room with floor-to-ceiling glass needs more cooling than a shaded south-facing bedroom of the same floor area. Ceiling height matters too — a 2.7m raked ceiling in an open-plan living area holds noticeably more air volume than a standard 2.4m bedroom, and that extra volume needs cooling as well.

How the Numbers Actually Work

The trade rule of thumb in Australia sits around 120 watts of cooling capacity per square metre of floor area, before you adjust for sun, occupancy, and ceiling height. That's not arbitrary — it comes from how much heat a typical room gains through walls, windows, and roof in an average Australian summer.

Say you've got a 4.5m × 3.6m bedroom. That's 16.2 m² of floor area. At 120W per m², base load works out to roughly 1,944 watts, or 1.94kW. Add the standard adjustments for room type and sun exposure and you land somewhere between 2.0kW and 2.5kW — which is exactly the bracket most 2.5kW split systems are built for.

Kitchens run hotter than other rooms because cooking appliances throw off extra heat, so the calculator above applies a 15% loading to kitchen results. Open-plan living and dining areas get a smaller loading because they're larger but not as heat-intensive room for room.

Why a Bigger Unit Isn't Always Better

Oversizing feels like the safe bet, but it backfires. An aircon that's too powerful for the room cools the air fast and shuts off before it's properly removed the humidity. You end up with a room that hits temperature quickly but still feels damp and uncomfortable — and the compressor short-cycling like that wears it out faster too.

There's a specific number worth knowing here: most manufacturers design their compressors for a minimum 10-minute run cycle. Undersize the room load and oversize the unit, and you can end up well under that, which is genuinely bad for the equipment's long-term reliability.

BTU vs kW — Which One Do You Actually Need?

Australian retailers list cooling capacity in kW. Some imported or older units, and most online forums, talk in BTU/h. The conversion is simple: 1 kW = 3,412 BTU/h. So a 2.5kW unit is roughly 8,530 BTU/h, and a 7kW unit is close to 23,900 BTU/h. The calculator above gives you both, so you're covered whichever spec sheet you're reading.

A Real Scenario

Take an open-plan living/dining/kitchen space, 7m × 5m, with a 2.7m ceiling and big sliding doors facing west. Floor area is 35 m², which on the base rate alone is already over 4kW. Add the western sun exposure, the open-plan loading, and the extra ceiling height, and you're realistically looking at a 6.0–7.0kW unit, maybe even considering a ducted system depending on layout.

Compare that to a south-facing 3m × 3m study with one small window. That's 9 m² of floor area — under 1.1kW on paper. In practice, no manufacturer makes a unit that small for a split system, so you'd size up to the smallest available split, typically 2.0kW, and accept it's oversized for the room. That's fine; a slight oversizing in a tiny, well-shaded room rarely causes the short-cycling problem described above.

Getting the Sun Exposure Rating Right

Don't underestimate this one. A west-facing room in summer can run 3–4°C hotter at peak afternoon heat than an identical south-facing room. If you're unsure which way your room faces, check it on a map app rather than guessing — getting this wrong throws the whole calculation out.

FAQs

How many BTU do I need for a bedroom?

A typical 12–16 m² bedroom with average sun exposure needs roughly 7,000–8,500 BTU/h, which lines up with a standard 2.0–2.5kW split system. Larger main bedrooms or ones with poor shading can push past 9,000 BTU/h.

What size split system do I need for a 20 square metre room?

At the base 120W/m² rate, 20 m² works out to around 2.4kW before adjustments. With average sun exposure and standard ceiling height, a 2.5kW–3.5kW unit usually covers it comfortably.

Is it better to oversize or undersize an air conditioner?

Neither extreme is good, but undersizing is the more common real-world mistake and the less forgiving one — an undersized unit runs constantly and never quite reaches the set temperature. A small amount of oversizing is more tolerable than a large shortfall.

How do I convert kW to BTU for an air conditioner?

Multiply the kW figure by 3,412 to get BTU/h. For example, 3.5kW × 3,412 = 11,942 BTU/h.

Does ceiling height affect aircon sizing?

Yes. Cooling load is based on air volume, not just floor area. A room with a 3m ceiling has 25% more air to cool than the same floor area at 2.4m, and that needs to be reflected in the unit's capacity.

Do I need a bigger unit for a room with lots of windows?

Generally yes, especially if those windows face north or west. Glass lets in far more heat than insulated wall, so a window-heavy room needs a higher sun exposure loading than the same room with small, shaded windows.

What size aircon do I need for an open-plan living and kitchen area?

Open-plan spaces are usually 30 m²+ and combine the heat loads of cooking, larger glass areas, and higher occupancy. Most homes in this bracket end up needing a 6kW–8kW system, or two separate split systems rather than one oversized unit.

Can one split system cool two rooms?

Only if there's an open doorway or archway between them and you treat it as one combined floor area when sizing. Closed-off rooms each need their own calculation — and ideally their own unit, since ducting cooled air through a doorway is inefficient.

Get the floor area and sun exposure right, run the numbers properly, and you'll land on a unit that actually matches the room — not one a salesperson talked you into. If the result above sits right on the boundary between two common unit sizes, it's worth getting a trade quote before buying, since wall insulation and window glazing vary enough between houses to shift things either way.