Type your exact room size, then set ceiling height, insulation, sun and climate. Get a recommended split system size with every standard size compared. Copy it, text it or save the PDF. Free, no email.
Aircon sizing
· backontools.com.au/free-tools/aircon-sizing-calculator
Ceiling height
Insulation
Sun exposure
Climate
2.5kW recommended split
Calculated load: 2.5kW for a 20m² room, rounded up to the nearest standard size.
Rate used at your settings: 125 W/m² (base 125 W/m² adjusted for ceiling, insulation, sun and climate).
| Split size | Room area it suits at your settings | At your room |
|---|---|---|
| 2.5kW | up to ~20m² | Recommended |
| 3.5kW | up to ~28m² | Bigger than needed |
| 5.0kW | up to ~40m² | Bigger than needed |
| 7.1kW | up to ~57m² | Bigger than needed |
| 8.0kW | up to ~64m² | Bigger than needed |
| 9.4kW | up to ~75m² | Bigger than needed |
Guide only. A proper heat load calculation covers glazing, orientation, occupancy, ducting and the specific unit's rated capacity at your design temperature. Final sizing is the licensed installer's call after seeing the room, not this page.
Your numbers stay on this device and come back next time. Nothing is uploaded.
Two identical 20m² rooms can need completely different units. One has 2.4m ceilings, decent insulation and a shaded window in Melbourne, and a 2.5kW split handles it easily. The other has 3m ceilings, no wall insulation and full western sun in Brisbane, and the honest answer is 3.5kW or even 5kW. Retailers quoting off floor area alone split the difference, and someone ends up with a unit that runs flat out all summer.
Slide the settings around and watch the W/m² rate move. The table shows what floor area each standard size genuinely suits at your conditions, so you can walk into the store, or price a supply and install job, already knowing the answer.
The common rule of thumb is 120 to 150 watts of cooling per square metre of floor area. This calculator starts at 125 W/m² and adjusts for ceiling height, insulation, sun exposure and your climate. A well insulated, shaded Melbourne bedroom lands near 100 W/m², while a west-facing Brisbane living room with high ceilings can push past 170 W/m². Same floor area, very different unit.
A little headroom is fine, a lot is not. A heavily oversized split cools the air fast, then short cycles on and off before it has pulled the humidity out, so the room feels cold and clammy and the compressor wears faster. Going one standard size up from the calculated load is the normal move. Going three sizes up because bigger feels safer usually makes the room less comfortable, not more.
Because the unit is fighting the heat coming through your walls, windows and roof, and that load depends on what is outside. A 40 degree Darwin afternoon pushes far more heat into a room than a 30 degree Hobart one, so the same room needs more capacity up north. The climate buttons shift the rate up or down about 15 percent, which is roughly one standard size on a mid sized room.
For a single bedroom or study, this gets you very close. For open plan areas, rooms with big glass, two storey homes or ducted systems, get a proper heat load assessment. Glazing, orientation and airflow between spaces move the answer in ways a room-size calculator cannot see. Use this page to sanity check a quote, not to replace one.
BackOnTools runs the rest of the job from your phone: the quote, the booking, the invoice and the chase for payment, with your price book behind it.
Free account, no card: 20 jobs a month, one login, quotes, invoices, scheduling and clients.
