How to Choose the Right Air Conditioning Unit for Your Room

Start with BTU, not brand

Air conditioner cooling capacity is measured in BTU (British Thermal Units) per hour. The single most common buying mistake is choosing a unit by price or looks first and worrying about capacity later. Get the BTU right and almost any reputable brand — Samsung, Toshiba, LG — will serve you well.

As a working rule of thumb for UK homes, allow roughly 120–150 BTU per square metre of floor area for a standard-height, reasonably insulated room. A 20 m² double bedroom typically needs around 9,000–12,000 BTU; a 30–35 m² open-plan living space usually lands at 16,000–18,000 BTU.

Adjust for sunlight, insulation and people

The floor area is only the starting point. Add capacity for a south-facing room or large unshaded windows (strong solar gain can add 10–20%), for poor insulation or rooms above a hot space like a kitchen, and roughly 500–600 BTU for every person beyond two who regularly occupies the room.

Conversely, a well-insulated, shaded north-facing room can often step down a size without any comfort loss.

Don't oversize — short-cycling is real

An oversized unit reaches the set temperature too quickly, switches off, then switches on again minutes later. This short-cycling leaves the air clammy because the unit never runs long enough to dehumidify properly, and it wears the compressor out faster. Slightly undersized and running steadily beats oversized and cycling.

Heat pump models earn their keep in the UK

Nearly all modern split systems are reversible heat pumps, meaning they heat as well as cool. In the UK climate that doubles the value: a 12,000 BTU unit can heat a bedroom or home office far more efficiently than a plug-in electric heater, because it moves heat rather than generating it.

Quick sizing shortcuts

Up to ~18 m²: 9,000 BTU. Around 18–25 m²: 12,000 BTU. Around 25–35 m²: 16,000 BTU. Larger open-plan spaces: 24,000 BTU. Always adjust up or down using the factors above — and when in doubt, use our free AC size calculator, which works it out from your room dimensions, sunlight, insulation, occupancy and pipe run.