Air conditioner size calculator: methodology

Method behind the Air conditioner size calculator · Last checked 2026-08-28

How the size is worked out

The calculator multiplies floor area by a watts-per-square-metre figure for your National Construction Code climate zone, then applies adjustment factors for ceiling height, insulation, window exposure, roof position, room use and occupants. The result is rounded up to the nearest standard split-system class: 2.5, 3.5, 5, 6, 7.1, 8 or 9.4 kW.

Base figures by NCC zone: zone 1 (Darwin, Cairns) 150 W/m²; zone 2 (Brisbane, coastal Queensland) 130; zone 3 (Alice Springs, inland north) 150; zone 4 (Dubbo, Mildura) 130; zone 5 (Sydney coast, Perth, Adelaide) 120; zone 6 (Melbourne, western Sydney) 110; zone 7 (Canberra, Hobart) 100; zone 8 (alpine) 110. In zones 6–8 the calculator also shows a heating figure 10–30 percent higher, because winter drives the choice there.

Adjustments: ceiling height divided by 2.4 m; insulation good ×0.85, average ×1.0, poor ×1.2; windows small/shaded ×0.9, average ×1.0, large or north/west-facing unshaded ×1.15; top floor or uninsulated roof ×1.1; kitchen or open plan ×1.1; plus 3 percent per occupant beyond two.

Where the numbers come from

The 100–150 W/m² band is an industry rule of thumb, not a government standard. CHOICE's worked examples for a 30 m² room (Darwin 2.9 kW, Perth 3.0, Brisbane 2.6, Sydney 2.3, Hobart 1.8) imply roughly 60–100 W/m² for an insulated room; Fujitsu General Australia's guide allows "around 0.15 kW per square metre"; Solar Choice uses 0.125 kW/m² with climate adjustments from Hobart +20 percent to Darwin +25 percent. The calculator's zone figures sit inside that range and lean toward the higher end, because undersizing a unit that runs flat out on a 40 °C day costs more in the long run than a half-size step up.

Neither energy.gov.au nor YourHome publishes a watts-per-square-metre figure; both point to the AIRAH FairAir calculator, which uses design temperatures from AIRAH DA09 and room-by-room construction inputs. If you are sizing a whole house, ducted system or a room with unusual glazing, use FairAir or a heat-load calculation from an installer. This tool is for a single room with a wall split.

Limitations

Rule-of-thumb sizing cannot see your glazing type, shading, wall construction or how many people and appliances heat the room. Two rooms of the same size in the same suburb can need different units. CHOICE's advice on the tie-break is to choose the slightly larger class; an oversized inverter unit short-cycles and dehumidifies poorly, so do not go up two classes.

Test vectors (for the build)

Data table: climate-zones.csv (CC BY 4.0).