What size aircon do I need?
There is no useful answer to this question without the room in front of you, and any figure quoted over the phone is a guess. This guide explains what actually decides the size, so you can describe your room well enough for a contractor to advise properly.
Why floor area is only the start
Floor area is the usual shorthand, and it is not wrong, it is just incomplete. Two rooms of the same size can need different capacity if one has a high ceiling, a wall of glass, or a roof that bakes in the afternoon sun.
The published Johannesburg guides give capacity bands against room size (source: Aircon Installers Johannesburg), and bands are exactly what they are: a starting point for a conversation with someone who measures the room. Treat them as a sanity check on a quote rather than the answer itself.
The things that change the answer
The heat load is the real question. Every person in the room adds heat, every light and every screen adds heat, and glass facing the afternoon sun adds a great deal. A west-facing living room with a big window and four people in it at night is a heavier job than a same-sized bedroom with one small window that faces south.
The building matters too. An upstairs room under a concrete roof holds heat far longer than a ground-floor room with a shaded ceiling. A room with an open passage or a stairwell beside it will never hold its temperature well, because the air simply moves away. Those are the details that turn a band into a decision.
How sizing is actually done
A contractor who sizes properly asks how big the room is, how high the ceiling is, how much window there is and which way it faces, how many people use it and what equipment is in it. From that they work out the heat load and pick a unit that covers it with a little in reserve.
That reserve matters, because a unit working at full stretch on the hottest day is a unit working near its limit. A small margin also means the unit reaches temperature quickly and then throttles back, which is quieter and cheaper to run than one that never quite gets there.
Rooms that are hard to size
Some rooms resist an easy answer. A loft, or a room with a cathedral ceiling, holds far more air than its floor area suggests, and that air stratifies, so the part you actually occupy is the last part to cool down.
An open-plan living area joined to a kitchen and a passage is another difficult one, because the cooled air leaves the space as fast as the unit makes it. A west-facing room with a large window takes the full afternoon heat at exactly the time you want to use it. In each case the answer may be a larger unit, a second unit, or a plan that treats the space as more than one room.
What happens when the size is wrong
A unit that is too small runs flat out and still leaves the room warm on the worst days. It is noisy, it wears faster, and on a hot afternoon it dehumidifies poorly because it never cycles long enough to pull moisture out.
A unit that is too large is the quieter mistake, and it is still a mistake. It cools the room quickly, then stops, then starts again, and that short cycling is poor at removing humidity and hard on the compressor. A room that feels cold and clammy is often a room with too much aircon rather than too little.
Frequently asked questions
Can I size the aircon myself?
You can estimate, and the published bands will get you close. The value a contractor adds is measuring the room and allowing for the ceiling, the glass, the sun and the people.
Does a bigger unit cool faster?
It cools faster and then cycles on and off, which removes humidity poorly and is hard on the compressor. A correctly sized unit runs more steadily and is usually more comfortable.
Does ceiling height matter?
Yes. A high ceiling is more air to cool, and an upstairs room under a hot roof needs more capacity than the same floor area downstairs.
Can one unit cool two rooms?
Only if the rooms are open to each other. A closed door stops the air, and a multi-split with a unit in each room is the honest answer where the doors are shut.