The short answer
Most bedrooms and smaller living rooms suit a unit of about 2.0 to 3.5 kW, and large open-plan spaces often need 5.0 kW or more. Sun, glazing, insulation and ceiling height can matter as much as floor area.
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Air conditioners are sold by their output in kilowatts: how much heat the unit can move out of a room on a hot day, or into it on a cold one. Two rooms with the same floor area can need quite different sizes, and getting the size roughly right does more for your comfort than fitting the biggest unit available.
Typical sizes for typical rooms
As a very rough starting point, for a room with average insulation, a ceiling of about 2.4 metres and ordinary double glazing:
| Floor area | Typical room | Size often fitted | BTU (approx.) |
|---|---|---|---|
| Up to about 15 m² | Most bedrooms, study | 2.0–2.5 kW | 7,000–9,000 |
| About 15–25 m² | Large bedroom, living room | 2.5–3.5 kW | 9,000–12,000 |
| About 25–35 m² | Large living room | 3.5–5.0 kW | 12,000–18,000 |
| About 35–50 m² | Open-plan kitchen-diner | 5.0–7.0 kW | 18,000–24,000 |
Treat the table as a starting point, not an answer. A room near the top of its band, or with several of the features below, may need the next size up, while a shaded, well-insulated room may manage with the next size down. If you're comparing units sold in BTU, kW and BTU explained shows how the two convert.
The kilowatt figure is the heat the unit moves, not the electricity it uses. A 3.5 kW unit typically draws much less than 3.5 kW from the mains.
What makes a room need more or less
Floor area is only one part of the picture. Each of these can push the size up or down:
- Sun. South- and west-facing rooms usually gain more heat than north-facing ones. West-facing rooms take the low afternoon sun when the day is already at its warmest.
- Glass. Large windows, patio doors and roof windows let in more heat. Conservatories and glass extensions behave very differently from ordinary rooms and need assessing on their own terms.
- Insulation. A room under a thinly insulated roof, or with uninsulated walls, gains heat faster in summer and loses it faster in winter.
- Floor level. Top-floor rooms and loft conversions usually run hotter than rooms downstairs.
- Ceiling height. A high ceiling means more air to deal with, so floor area alone understates the job.
- People and equipment. A person sitting still gives off roughly 100 watts of heat. Computers, screens and cooking add more, so a home office or kitchen-diner can need more than a bedroom of the same size.
- Open doorways. If a room opens onto a hall, a stairwell or another room, the unit ends up working on part of that space too.
Why bigger isn't better
It's tempting to size up to be safe, but an oversized unit often makes a room less comfortable rather than more.
- It runs in short bursts. Inverter units slow down to match what the room needs, but every unit has a minimum output. If the room needs less than that, the unit keeps switching off and on again, which is less efficient and harder on the compressor.
- It takes out less moisture. A unit that reaches the set temperature quickly spends less time drawing water out of the air, so the room can feel cool but clammy.
- It's bigger outside. Larger outdoor units are usually louder, which affects where the unit can go and whether it meets noise limits at a neighbour's window.
An undersized unit has the opposite problem. On the hottest days it runs flat out, is noisier, and may not bring the room down to the temperature you've set.
If you want it to heat as well
Most split systems heat as well as cool, and the heating need can decide the size. In a poorly insulated room, keeping warm on a cold day can take more output than keeping cool on a hot one.
The rated heating output on a specification sheet is measured in mild conditions, and the real output falls as it gets colder outside. If you plan to rely on the unit for heating through winter, the installer should check how much heat it gives at low outdoor temperatures, not only the headline figure. If radiators will still do most of the heating, the cooling need usually decides the size.
How an installer should size it
A good installer will look at the room rather than just ask for its floor area. Expect them to:
- measure the room, including the ceiling height
- note the windows, which way they face and whether they get direct sun
- ask about insulation and what's above the room
- ask how you use the room and whether you want heating as well as cooling
- explain the size they recommend and why
Some installers use heat-load calculation software and others work from experience, but either way you should get a clear reason for the size. If two quotes suggest very different sizes for the same room, ask each installer to explain the difference. There are more questions to ask an installer in a separate guide.
If you want to cool several rooms, each room is sized on its own. The indoor units can be separate systems or share one outdoor unit as a multi-split system, with the outdoor unit chosen to suit the combination.
Related guides
- kW and BTU explainedkW and BTU per hour both measure an air conditioner's capacity, meaning how much heat it can move. One kW is about 3,412 BTU per hour, so a 3.5 kW unit is roughly a 12,000 BTU unit.
- Why some rooms overheatA room overheats when heat gets in or builds up faster than it can escape. The usual causes are sunshine through large or west-facing windows, a roof directly above, heat from people and equipment, and too little ventilation.
- Multi-split systemsA multi-split system runs two or more indoor units, usually in different rooms, from one outdoor unit. Each room has its own controls, and your home needs only one outdoor unit rather than one for each room.
- Questions to ask an installerAsk how they sized the system, where both units and the pipes will go, whether it meets planning and noise rules, what the quote includes, which F-gas and electrical qualifications they hold, and what warranty and servicing come with it.