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What size portable air conditioner do I need?
Pick too few BTU and it never cools the room; too many and you have overpaid for a clammy result. Here is how to get it right first time.
BTU (British Thermal Units) is the measure of a portable air conditioner's cooling power. The single most common mistake UK buyers make is guessing — and guessing wrong is expensive in both directions. Undersize it and the unit roars at full power and still loses to a hot afternoon. Oversize it and you have paid more, it is bulkier, and ironically the room can feel damp because the unit cools so fast it does not run long enough to pull out humidity.
The rule of thumb
For UK rooms, budget around 340 BTU per square metre of floor area, assuming standard 2.4 m ceilings. Then adjust up for anything that adds heat. Here is the shortcut table most people can use as-is:
| Room size | Typical use | BTU you want |
|---|---|---|
| Up to 18 m² | Small bedroom, study | 7,000–9,000 BTU |
| 18–28 m² | Double bedroom, home office | 9,000–12,000 BTU |
| 28–40 m² | Living room, open-plan | 12,000–16,000 BTU |
| 40 m²+ | Large open-plan, knock-through | 16,000 BTU+ (or two units) |
When to size up
Add roughly 10–20% to the BTU figure if your room is any of these:
- South or west facing with big windows that get direct afternoon sun.
- Top-floor or loft rooms that bake under the roof.
- Kitchens or rooms with lots of electronics (each adds heat).
- Rooms with several people in them regularly — bodies are heat sources too.
If you are between two sizes and the room ticks any of the above, go up. Our on-site BTU calculator bakes these adjustments in — enter your room size and sun exposure and it gives you a target rating.
Matching real models to room sizes
To make it concrete, here is how a few of the models we track map onto rooms:
- Small bedroom: a 9,000 BTU unit like the Blyss 9000 or MeacoCool MC Pro 9000.
- Double bedroom / office: 12,000 BTU — the Meaco Cirro 12000 is the quiet pick.
- Living room: 14,000–16,000 BTU, such as the Meaco Cirro 14000 or Meaco Pro 16000.
Two things that matter as much as BTU
Noise, if it is going in a bedroom — look for quoted figures around 45 dB on quiet mode. And the window kit: the unit can only cool if the hot exhaust air actually leaves the room, so a good seal on the window adaptor is not optional. A perfectly sized unit venting into a poorly sealed window will still disappoint.
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Set a free stock alert →Figures are approximate and change with energy prices, stock and season. This guide contains affiliate links — we may earn a small commission at no cost to you, and it never affects what we recommend.