How many BTU do I need for my room?

The rule of thumb: BTU per square metre

BTU (British Thermal Units) is the measure of cooling power. As a starting point, allow around 350 to 400 BTU per square metre, assuming a normal ceiling height and average insulation. Multiply your room's floor area by that figure and you have a sensible target. For a 20 m² room, for example, 20 × 350–400 gives roughly 7,000 to 8,000 BTU, so a 9,000 BTU unit is the practical choice. Treat the result as a guide, not an exact science — it's the right ballpark, and the adjustments below refine it.

Room-size table

Mapped onto the units most people actually buy, the rule of thumb works out like this:

• Up to 14 m² (small bedroom, home office): around 5,000 BTU — such as the Honeywell HF09.
• Up to 25 m² (standard living room or bedroom): around 9,000 BTU — such as the MeacoCool MC 9000 or Pro Breeze 9,000 BTU.
• Up to 30 m² (larger living room): around 9,800 BTU — such as the De'Longhi Pinguino.
• 30 to 35 m² (large or open-plan room): around 12,000 BTU — such as the Russell Hobbs 12,000 BTU.

If your room falls between two classes, the safer choice is usually the smaller one — provided none of the factors below apply. A correctly sized unit cycles quietly and economically; an oversized one short-cycles and is louder than it needs to be.

Factors that raise the BTU you need

The square-metre rule assumes an average room. Several things mean you should step up a class:

• Large windows facing south or west, which let in a lot of afternoon sun.
• Loft and top-floor rooms, which take heat through the roof.
• Poor insulation or single glazing, which let heat in faster than the unit can remove it.
• High ceilings, because you're cooling more air than the floor area suggests.
• Several people, or heat-producing equipment like PCs, servers or a kitchen.

If just one of these applies, the rule of thumb is usually still fine. If two or more stack up — say a south-facing loft room with poor insulation — size up a class so the unit isn't forced to run flat out all day.

Why bigger isn't always better

It's tempting to buy the most powerful unit you can, but oversizing causes its own problems. An overpowered unit cools the room fast, then switches off — and because it cooled so quickly, it never runs long enough to dehumidify properly, so the air feels cold but clammy. It also short-cycles, switching on and off repeatedly, which is louder, harder on the compressor and less efficient. A unit matched to the room cools steadily, dehumidifies as it goes and runs quietly. Right-sizing isn't about caution; it's about getting better cooling and a more comfortable room.