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.