Answer four questions and get the cooling capacity in kW and BTU. Sizing matters: an undersized unit runs constantly and never gets there, an oversized one cycles on and off, dehumidifies badly and costs more than it should.
How this works: we use ~340 BTU per m² as the UK baseline for a standard-height room, then adjust for solar gain, ceiling volume, occupancy and appliance heat, and round up to the nearest standard unit size (2.0, 2.5, 3.5, 5.0, 7.0 kW). It's a solid guide for choosing a unit — a proper installer survey will fine-tune for insulation, glazing and aspect.
Air conditioner sizing is the decision everything else depends on, and both failure directions cost you. An undersized unit runs continuously on hot days without ever reaching the set temperature — you pay full running costs for a room that's still 28°C, and the compressor wears faster for the privilege. An oversized unit fails less intuitively: it blasts the room to temperature so quickly that it switches off before dehumidifying the air, then cycles on and off repeatedly — leaving the room cold but clammy, with more wear and higher bills than a correctly sized unit holding a steady output. The sweet spot is a unit rated at or slightly above your room's calculated load, which runs long, efficient, quiet cycles.
The model starts from roughly 340 BTU of cooling per square metre of floor area — a widely used baseline for UK rooms with standard ~2.4m ceilings — then adjusts for the factors that move real-world heat load: solar gain (a south-facing room needs 10–15% more; shaded rooms less), ceiling height (it's the air volume you're cooling, not just the floor), occupancy beyond two people, and appliance heat in kitchens and equipment-heavy rooms. The result is rounded up to the nearest standard unit size — 2.0, 2.5, 3.5, 5.0 or 7.0 kW — because that's how systems are actually sold. It's deliberately conservative and aligns with common UK installer rules of thumb; a professional survey will fine-tune for insulation, glazing and aspect, but you shouldn't end up more than one size class away from this estimate.
Special cases worth knowing: lofts and conservatories carry far more heat load than their floor area suggests — set sun exposure to "sunny" as a minimum and read our dedicated loft and conservatory guides. If the result points at 2.5 kW or below, a well-sized portable is viable — see our current picks and don't skip sealing the window kit properly. Above ~3.5 kW, a fitted split system wins decisively on noise and running cost: benchmark the fitted price with the installation cost calculator, check the pence-per-hour in the running costs guide, then compare free quotes from certified installers.
UK spec sheets mix both units: 1 kW ≈ 3,412 BTU/h, so 9,000 BTU ≈ 2.6 kW, 12,000 BTU ≈ 3.5 kW, and 18,000 BTU ≈ 5.3 kW. Marketing sometimes rounds generously — when comparing units, trust the kW cooling capacity figure on the energy label over the headline BTU class.