This is not an AS/NZS 5141 heat-load calculation
Australia does have a standard here: AS/NZS 5141:2018, Residential heating and cooling systems, covering how these systems should be designed, selected and installed — and the way it sizes a system is a proper heat-load assessment of your actual rooms. What no standard prescribes is the watts-per-square-metre shortcut this page uses. Those bands are a rule of thumb published by installers and retailers, every Australian sizing tool we checked runs on the same rough method, and most present it as though it came from somewhere official. This one tells you which kind of number you are getting.
Treat the answer as a starting point for a conversation with an installer — and expect a good one quoting a whole house to do the heat-load work properly rather than take a number off a web page. It is not a specification, and nobody should order a ducted system off it.
The watts per square metre bands
| Climate | Where | Rate |
|---|---|---|
| Cool | Tasmania, Victorian highlands | 120 W/m² |
| Temperate | Melbourne, Sydney, Adelaide, Perth | 140 W/m² |
| Subtropical | Brisbane, northern NSW | 155 W/m² |
| Tropical | Darwin, Cairns, Townsville | 175 W/m² |
Multiply the room's floor area by the rate and you have watts. Divide by a thousand for kilowatts, which is how Australian units are sold.
Quick reference — kW by room size and climate
| Room | Cool | Temperate | Subtropical | Tropical |
|---|---|---|---|---|
| 15 m² bedroom | 1.8 kW | 2.1 kW | 2.3 kW | 2.6 kW |
| 20 m² bedroom / study | 2.4 kW | 2.8 kW | 3.1 kW | 3.5 kW |
| 30 m² lounge | 3.6 kW | 4.2 kW | 4.7 kW | 5.3 kW |
| 40 m² open living | 4.8 kW | 5.6 kW | 6.2 kW | 7.0 kW |
| 60 m² big open plan | 7.2 kW | 8.4 kW | 9.3 kW | 10.5 kW |
Standard Australian split sizes run roughly 2.5, 3.5, 5.0, 6.0, 7.1, 8.0 and 9.5 kW, so in practice you round up to the nearest one.
A worked example
A 5m × 4m lounge in Brisbane, 2.7m ceilings, west facing with two big sliding doors:
- Area: 5 × 4 = 20 m²
- Subtropical band: 20 × 155 = 3,100 W
- Ceiling 2.7m instead of 2.4m: 2.7 ÷ 2.4 = ×1.125 → 3,488 W
- High sun exposure: ×1.15 → 4,011 W
- Two large glass doors: ×1.10 → 4,412 W, so 4.41 kW
- Nearest standard unit up: 5.0 kW
The same room in Melbourne, 2.4m ceilings, south facing and shaded, works out at 20 × 140 × 0.93 = 2,604 W — a 3.5 kW unit. Same floor plan, one size apart. That is why the plain "square metres to kilowatts" charts get it wrong so often.
Bigger is not safer — oversizing is a real problem
Rounding up hard backfires, and here is how to explain that to a customer who wants the biggest unit on the wall.
- Short cycling. An oversized unit hits the set temperature fast, shuts down, and restarts a few minutes later. Compressors wear out on start-ups, not on running hours.
- It never dries the air. Dehumidification happens while the unit runs. A unit that only runs in short bursts leaves a room cold and clammy — exactly the complaint you get in Brisbane and Darwin, and exactly where people oversize the most.
- More money, twice. Dearer to buy, and inefficient at the part load it spends its whole life in.
Undersizing has its own problems — the unit runs flat out, never quite gets there, and costs a fortune on the hottest week of the year. The answer is not "go big", it is "get the number roughly right, then check it properly".
What a proper heat-load calculation does
A heat-load calculation sees the building, not just floor area and a climate label. It accounts for:
- Orientation — which way each wall and window faces, and when the sun hits it
- Glazing — area, single or double, tinted, shaded or bare
- Insulation — ceiling, wall and floor R-values, individually
- Infiltration — how much outside air leaks in, which is enormous in an old weatherboard and small in a new build
- Internal gains — people, cooking, a TV, lighting
For a single split in a bedroom, the rule of thumb is usually close enough to shop with. For a ducted system, do the heat load. Ducted is a lot of money, it is sized zone by zone, and a guess-and-round job produces rooms that never balance and an owner who is unhappy for a decade.
kW to BTU, for anyone reading a US spec sheet
Australian units are rated in kilowatts. American ones are in BTU per hour. 1 kW ≈ 3,412 BTU/h.
| BTU/h (US) | kW (Australia) | Nearest AU unit |
|---|---|---|
| 9,000 | 2.64 kW | 2.5 or 3.5 kW |
| 12,000 | 3.52 kW | 3.5 kW |
| 18,000 | 5.28 kW | 5.0 kW |
| 24,000 | 7.03 kW | 7.1 kW |
Worth knowing when a customer forwards you a link to something they found online.
Installing it is licensed work
Fitting the unit is regulated in three separate ways.
- Refrigerant. Handling refrigerant — connecting, charging, recovering, decommissioning — requires a refrigerant handling licence, and the business needs a refrigerant trading authorisation to buy and hold the gas. This is federal, it applies everywhere in Australia, and the penalties for working without it are serious.
- Electrical. The connection is electrical work and must be done by a licensed electrician. A hard-wired split or a ducted unit generally wants its own dedicated circuit, so on an older switchboard the job may be bigger than the customer expects.
- Condensate and mounting. Drains have to fall somewhere legal, and outdoor units near a boundary can run into noise rules that vary by state and council.
Doing the electrical side of someone else's install? Look at the switchboard before you quote and price that work separately. It is where these jobs blow out.
Sizing one for your own place? The number above is enough to shop with, but the rooms have to be seen before it is real — find an air-conditioning installer near you and ask them to check the sizing on site.
Sized it up. Now get the quote out before someone else does.
Yamate turns your site notes into a quote the customer can accept on their phone, then an invoice, then a payment — with GST worked out and the follow-ups chased for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberAir conditioner sizing FAQ
What size air conditioner do I need for a room in Australia?
Multiply the floor area by 120 to 175 watts per square metre depending on your climate — 140 W/m² suits Melbourne, Sydney, Adelaide and Perth. A 20 m² room in a temperate climate comes out around 2.8 kW, so you would look at a 3.5 kW unit.
Is there an Australian Standard for air conditioner sizing?
AS/NZS 5141:2018 covers residential heating and cooling systems — their design, selection and installation — and the way it sizes a system is a proper heat-load assessment. No standard prescribes the watts-per-square-metre shortcut used by online tools, including this one. Treat any web answer as a starting point rather than a specification.
Is it better to oversize an air conditioner?
No. An oversized unit short-cycles, wears the compressor on repeated start-ups, and never runs long enough to pull moisture out — so the room ends up cold and clammy. It also costs more to buy and runs inefficiently at part load.
How many BTU is a 3.5 kW air conditioner?
About 11,900 BTU per hour, since 1 kW is roughly 3,412 BTU/h. Going the other way, a 12,000 BTU American unit is about 3.5 kW.
Does ceiling height change the air conditioner size?
Yes — you are cooling volume, not floor area. A 2.7m ceiling is 12.5% more air than a standard 2.4m one, and a 3m ceiling is 25% more. The calculator above scales for it.
When do I need a proper heat-load calculation?
For any ducted system, and for anything unusual — big glazing, a poorly insulated old house, a west-facing room that bakes. A heat load accounts for orientation, glazing, insulation and air leakage, which a square-metre rule of thumb cannot see.
Who can install a split system in Australia?
Anyone handling refrigerant needs a refrigerant handling licence, and the business needs a refrigerant trading authorisation to buy the gas. The electrical connection must be done by a licensed electrician. Both apply everywhere in Australia.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. This is a rule-of-thumb guide, not a specification and not an AS/NZS 5141 heat-load calculation. The watts-per-square-metre bands used here are the shortcut Australian installers publish, not figures prescribed by a standard. A proper heat-load calculation accounts for orientation, glazing, insulation and air infiltration, and is what you should be doing for a ducted system or an unusual room. Installation involves licensed refrigerant handling and licensed electrical work.