Almost nobody chooses an aircon by capacity. People choose a brand, then a budget, then a System 3 or a System 4 because that is how many rooms they have. The capacity of each indoor unit — the BTU figure on the spec sheet — gets decided by whoever quotes, and the homeowner finds out whether it was right about two months later, in the middle of a hot afternoon.
That is backwards, because capacity is the one decision that cannot be fixed afterwards without changing the unit. Everything else — brand, controls, pipe routing, the servicing schedule — can be adjusted later. A unit that is the wrong size for the room stays the wrong size for its whole life.
What BTU actually measures
BTU per hour is a rate of heat removal. A 9,000 BTU unit and a 24,000 BTU unit do exactly the same thing; the bigger number simply moves heat out of the room faster. Singapore product listings usually abbreviate it to "9,000 BTU" or "9k", and the common residential classes step up from there — 9,000, 12,000, 18,000, 24,000 — with the exact figures varying slightly by brand and model.
What matters for your flat is not the number on its own but the number set against how much heat the room gains. Sizing is a balance: the unit has to remove heat at least as fast as the room takes it in, on the worst afternoon of the year, without being so oversized that it never has to work.
Why the chart on the internet gets Singapore wrong
Search for aircon sizing and you will find a table matching room area to a BTU figure. Those tables come from temperate climates and from an assumption that a room is a box with one small window and nobody in it.
A Singapore bedroom breaks that assumption in several ways at once:
- Sun exposure. A west-facing room takes several hours of direct afternoon sun through the glass. The same floor area facing east, or shaded by the next block, is a different room from the aircon's point of view.
- Which floor you are on. A top-floor flat gains heat through the roof slab all afternoon and keeps releasing it into the evening. A middle-floor unit sandwiched between two air-conditioned flats does not.
- Glass area. Floor-to-ceiling windows and full-height balcony doors are the largest single heat gain in many condo living rooms. Two rooms of identical size, one with a small window and one glazed across a whole wall, are nowhere near the same load.
- Ceiling height. The chart is priced in floor area, but the unit has to cool a volume. A maisonette, a loft, or a room with a raised ceiling holds noticeably more air than the floor plan suggests.
- People and equipment. Bodies produce heat. So do a gaming PC, a router cabinet, a large television and a kitchen that opens onto the living room. A living room used by five people on a Sunday is not the same load as the same room used by one person on a weekday.
- Whether the space is really one room. An open-plan living and dining area with no door between them is a single load on a single unit, no matter what the floor plan calls each half.
None of these appear on a floor-area chart, and in Singapore any one of them can move the answer by a whole capacity class.
Oversizing is the more common mistake — and the worse one
Most people assume the risk is buying a unit that is too small, so they round up. In practice the more frequent problem in Singapore flats is a unit that is too big for the room it serves, and the symptoms are easy to mistake for something else.
An oversized unit pulls the room down to the set temperature very quickly, then stops. It does that over and over, in short bursts. Two things follow. First, the room feels cold and clammy rather than cool and dry — because removing humidity takes running time, and a unit that keeps switching off never gets a long enough run to do it. Second, all that starting and stopping is harder on the compressor than steady running, and it undoes much of the point of paying for an inverter, which is designed to find a low speed and hold it.
So the complaint that arrives is usually not "my aircon is too powerful". It is "the room feels damp", or "it smells musty", or "my bill went up after I upgraded". Those symptoms have other causes too, which is exactly why the sizing question deserves settling before installation rather than after.
Undersizing looks different
A unit that is genuinely too small never reaches the set temperature on a hot afternoon. It runs continuously, the room sits a few degrees above where the remote says it should be, and the situation is at its worst between about 2pm and 6pm on the sunny side of the building.
The reason this is worth separating from a fault is that a dirty coil produces very similar symptoms. Before concluding a unit is undersized, it is worth ruling out the ordinary causes — we go through those in what to check when the aircon is not cold enough. A system that cooled the room properly two years ago and does not now has almost certainly not shrunk; something else has changed.
The part specific to multi-split systems
This is the piece most sizing guides leave out, and in Singapore it matters more than the room chart does.
A System 3 or System 4 is several indoor units sharing one outdoor condenser. The condenser has its own capacity, and it is usually not the simple sum of every indoor unit running flat out at the same time. Manufacturers publish the combination ratios their systems allow, and the practical effect is that adding a large indoor unit to a system changes what is left for the others when everything is running together — a Sunday evening with all bedrooms on, for example.
That is why the useful question is not "what size for this bedroom" but "what set of indoor units, on what outdoor unit, for the way this household actually uses the flat". Which rooms run at the same time, and which never do, changes the answer. We go through how the system count itself is chosen in System 3 vs System 4 vs System 5.
Questions to ask before you accept a quotation
You do not need to do the calculation yourself. You do need the person quoting to show that they did one.
- Which way does this room face, and did you allow for it? If the answer does not distinguish the west-facing bedroom from the shaded one, the quote was written off the floor plan.
- What capacity are you proposing for each indoor unit, in BTU? Get the figure in writing per room, not just "System 4".
- What outdoor unit, and what happens when every room runs at once? This is the combination-ratio question above.
- Is this the same size as what is there now, and if so, was the old one right? Replacing like-for-like copies the previous owner's decision, including their mistake.
- What happens if it turns out to be wrong? A straight answer here tells you a lot.
How we settle it
Our site survey is free, and for a sizing question it is genuinely the whole job: measuring the rooms rather than reading the floor plan, seeing which way the flat faces and what shades it, looking at the glass, checking what is already installed and what the existing piping and ledge will take, and asking how the household actually uses the space. Only then does a capacity per room mean anything.
If the outcome is a new installation, our prices are published in full: from $1,200 for a single split, $2,200 for a System 2, $2,800 for a System 3 and $3,500 for a System 4 on the standard inverter tier, with the premium tier and the full inclusions on the installation cost page and the general terms on our installation service page. What a standard installation does and does not include is set out in what is included in a standard aircon installation.
If you are replacing units in an older flat, the existing copper piping and trunking are part of the sizing conversation too, because they limit what can practically be fitted — that is covered in replacing aircon in an older HDB flat.
The short version
Capacity is decided by the room, not by the floor plan and not by the number of bedrooms. In Singapore the orientation, the floor you are on, the glass and the ceiling height move the answer more than the square metres do. Oversizing is the commoner error and shows up as a damp, clammy room rather than an obviously wrong one. And on a multi-split system, the indoor units have to be chosen as a set, not one at a time.
Get the sizing checked before you commit
Tell us the flat type, which rooms you want cooled, and which way the main windows face. If you already have a quotation, send us the capacities on it — we will tell you plainly whether they make sense for those rooms.
WhatsApp 9107 2601 for a free sizing survey →
Related reading: System 3 vs System 4 vs System 5 · what a standard installation includes · inverter vs non-inverter