How Many BTU Do You Need? (Portable AC Sizing Guide)

By the Arctic Pick editorial team · Updated 2026 · How we test & score

BTU (British Thermal Units) is the cooling-power rating of a portable air conditioner, and matching it to your room is the most important decision you'll make. Here's a simple guide to getting it right.

The simple BTU guide

As a rule of thumb for UK rooms: around 5,000-7,000 BTU for a small bedroom or home office, 8,000-10,000 BTU for an average bedroom or living room, and 12,000-14,000+ BTU for a large or open-plan space. Add a bit more if the room is south-facing, top-floor, has large windows or lots of electronics, as these all add heat. If you're between sizes, round up rather than down.

Why getting it right matters

An undersized unit (too few BTU) will run continuously and never bring the room to a comfortable temperature, wasting money for poor results. An oversized unit cools quickly but cycles on and off, which uses more energy and can leave the air feeling cold and damp rather than comfortable. Matching the BTU to the room gives the best balance of comfort, running cost and noise - it matters more than the brand.

Our top picks

Frequently asked questions

How many BTU to cool a bedroom?

Around 5,000-9,000 BTU suits most bedrooms, depending on size and how sunny the room is. A small box bedroom needs less; a large or south-facing one needs more.

Is more BTU always better?

No - an oversized unit cools too fast and cycles off before removing humidity, leaving the air clammy, and it costs more. Match the BTU to the room rather than buying the biggest you can.

Bottom line

Our top pick is the Portable Air Conditioner (our score 9.6/10) - A practical portable air conditioner (7000 BTU, 4-in-1) for proper, portable cooling around the home..

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