# How Many BTU Do I Need? Air Conditioner Room Size Guide (2026)

> A simple BTU calculator by room size in m² for portable air conditioners, plus the factors (sun, top floor, people) that mean you should size up. 2026 guide.

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BTU (British Thermal Units) measures an air conditioner's cooling power. Too few and it can't keep your room cool; too many wastes money and makes more noise. Here's the simple version.

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## BTU by room size

A good rule of thumb is roughly **350–400 BTU per m²**. Use this chart:

Room size | Recommended BTU | Typical room |

Up to 15 m² | 7,000–9,000 BTU | Small bedroom, study |

15–25 m² | 9,000–12,000 BTU | Bedroom, home office |

25–35 m² | 12,000–14,000 BTU | Living room |

35 m²+ | 14,000+ BTU / monobloc | Open-plan, large lounge |

## Add BTU if…

- **The room gets strong afternoon sun** — add ~10%.

- **It's a top-floor or attic room** — heat rises, size up.

- **Several people or a kitchen/electronics** in the space — each adds heat.

- **High ceilings** (above ~2.7 m) — you're cooling more air volume.

## Don't oversize "to be safe"

A too-powerful unit short-cycles (blasts, stops, repeats), which cools unevenly, runs louder, and costs more. Match the room and only bump up for the factors above.

**Know your number?** Filter portable ACs by BTU on Amazon.de:

See portable ACs on Amazon.de →

For a bedroom, also check the noise rating — see [best portable AC for a bedroom](https://getecoback.com/en/guide/best-portable-air-conditioner-for-bedroom.html). Got tilt-and-turn windows? Read [how to vent a portable AC with tilt-and-turn windows](https://getecoback.com/en/guide/portable-ac-tilt-and-turn-windows.html) first.

**Related guides**
[Full portable AC buying guide →](https://getecoback.com/en/guide/best-portable-air-conditioner-europe-heatwave.html)
[Portable AC vs air cooler →](https://getecoback.com/en/guide/portable-ac-vs-air-cooler.html)
[Cheapest way to cool a room without installation →](https://getecoback.com/en/guide/cheapest-way-to-cool-a-room-without-installation.html)

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