What size air conditioner do I need for a room?

Rules of thumb based on square metres are common and they are the reason so many Australian homes have the wrong sized unit. Floor area is one input among several, and the others often matter more.

What a proper sizing considers

• Climate zone, which changes the design temperatures the unit has to cope with.
• Ceiling height, since the volume matters rather than the floor.
• Glazing: area, orientation, whether it is shaded, and whether it is single or double glazed.
• Insulation in the ceiling and walls, and whether the floor is slab or suspended.
• How many people typically use the room and what else generates heat in it.
• Whether doors are usually open to adjoining spaces, which effectively enlarges the room.

Why oversizing is not the safe option

An oversized unit reaches the set temperature quickly and then cycles off. In cooling that means it never runs long enough to remove humidity, so the room feels cold and clammy rather than comfortable. Short cycling also wears the compressor and uses more energy than a correctly sized unit running steadily. Bigger is not safer, it is a different fault.