What size air conditioner do I need?

Room size to split system size

Rule of thumb: about 120 to 150 watts per square metre of floor.

Room size calculator

The calculator needs JavaScript. The table above gives the same answer.

What changes the size

• Afternoon sun. West-facing rooms and big windows heat up. Add about 15%.
• High ceilings. More air to cool. A 2.7 m ceiling needs about 12% more than 2.4 m.
• No ceiling insulation. Heat comes straight through the roof. Add about 15%.
• Open to the kitchen. Cooking adds heat. Add about 10%.

Why bigger isn't better on the coast

An air conditioner cools the air and dries it. A unit that's too big drops the temperature fast and switches off before it has pulled the moisture out. In our humid summers that leaves the room cold and clammy.

Too small is no better. It runs flat out on hot days and still can't keep up.

Two worked examples

Bedroom, 3.5 m × 4 m. That's 14 m². At 120 W per m² it needs about 1.7 kW, so a 2.0 kW split system covers it, even with afternoon sun.

Open-plan living, 6 m × 8 m. That's 48 m². At 125 W per m² it needs 6.0 kW, plus 10% for the kitchen, so about 6.6 kW. Pick a 7.1 kW split system.

Quick answers