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This guide explains what size air conditioner you need. BTU is a measure of cooling power. As a rough guide, 5,000 to 7,000 BTU suits a small bedroom, 7,000 to 9,000 a double bedroom, 9,000 to 12,000 a living room and 12,000 or more large or open-plan rooms. An oversized unit costs more to run and removes humidity poorly.
- Small bedroom: 5,000 to 7,000 BTU.
- Double bedroom: 7,000 to 9,000 BTU. Living room: 9,000 to 12,000 BTU.
- Add about 10 to 20% for south-facing, top-floor or glass-heavy rooms.
- Bigger is not better: an oversized unit costs more and removes humidity poorly.
An air conditioner's power is measured in BTU. A bedroom needs about 7,000 BTU and a living room about 10,000. Do not buy one that is too big for the room, because it costs more to run and does not dry the air properly.
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The short version
- BTU is a measure of cooling power. Match it to the room, not to your budget.
- Rough guide: 5,000 to 7,000 BTU for a small bedroom, 7,000 to 9,000 for a double bedroom, 9,000 to 12,000 for a living room, 12,000+ for large or open-plan rooms.
- Add about 10 to 20% for south-facing, top-floor, glass-heavy or busy rooms.
- An oversized unit costs more to run and removes humidity poorly, so bigger is not better.
The most common mistake with portable air conditioners is buying on price or power and ignoring room size. Too little cooling and the unit never catches up; too much and you waste money. BTU is the number that matters, and matching it to your room is simple once you know the rule of thumb.
What does BTU mean?
BTU, or British Thermal Units, measures how much heat an air conditioner can remove from a room in an hour. A higher BTU figure means more cooling power. The job is to pick enough BTU for your room without going over, because an oversized unit short-cycles, which cools the air but leaves it damp and uses more electricity.
What size air conditioner for my room?
| Room size | Room type | Recommended BTU |
|---|---|---|
| Up to 14 m² | Small bedroom, study | 5,000 to 7,000 |
| 14 to 20 m² | Double bedroom | 7,000 to 9,000 |
| 20 to 28 m² | Living room | 9,000 to 12,000 |
| 28 to 40 m² | Large or open-plan | 12,000 to 14,000+ |
When to size up
The table assumes an average room. Add roughly 10 to 20 percent more BTU if the room faces south or west and catches the afternoon sun, sits on the top floor under a warm roof, has large or numerous windows, or is often full of people. Each of those adds heat the unit has to shift, so a room at the top of a size bracket with several of these factors is better served by the next size up.
Why bigger is not better
It is tempting to buy the most powerful unit you can afford, but an oversized air conditioner cools the air so fast that it switches off before it has pulled much moisture out. You end up with a cold, clammy room, higher running costs, and more noise. Right-sizing is cheaper and more comfortable. Once you have your number, see our pick of the best portable air conditioners and what they cost to run.
Common questions
What size air conditioner do I need for a bedroom?
A typical double bedroom of 14 to 20 square metres is well matched to a 7,000 to 9,000 BTU portable air conditioner. A small bedroom needs less, around 5,000 to 7,000 BTU.
How many BTU do I need for a living room?
A living room of 20 to 28 square metres usually needs 9,000 to 12,000 BTU. Large or open-plan rooms above 28 square metres want 12,000 BTU or more.
Is a higher BTU air conditioner always better?
No. An oversized unit costs more to buy and run, and it cycles on and off too quickly to remove humidity properly. Matching the BTU to the room is more effective and cheaper.
When should I size up?
Add roughly 10 to 20 percent more BTU if the room faces south, is on the top floor, has large windows, or is regularly full of people, since all of these add heat the unit has to remove.
The portable air conditioners we'd buy
How Look Into made this guide: desk research, not a hands-on test. We shortlist on UK availability, compare published specifications and warranty terms, and read verified UK owner reviews, discounting incentivised ratings. Prices in the comparison table are pulled live from Amazon UK. Look Into has not handled these products: every figure comes from the maker, the listing or a named source, read on the date shown.
As an Amazon Associate, Look Into earns from qualifying purchases. Product prices and availability are accurate as of 15 November 2026 5:39 am and are subject to change.
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