Air con
What size air conditioning do I need for a room?
3 min read · Last updated · Reviewed for accuracy before publishing
Allow about 0.1 kW per square metre as a starting point. Daikin's guide puts a well-insulated 20 m² bedroom at about 2 kW and a 35 m² living room at about 3.5 kW, with large south or west windows adding 10–20%. BestBuilders says an undersized unit runs flat out and never cools the room, while an oversized one short-cycles and wastes money. We confirm the size at the survey.
Why floor area isn't enough
Two rooms the same size can need very different units. A north-facing bedroom with modest windows is easy; a south or west-facing living room with big glazing takes a lot of afternoon solar gain and needs more capacity. Daikin adds 10–20% for large south or west windows.
A conservatory is the extreme case — mostly glass, so heat load is high. We size to the glazing and aspect, not a square-metre rule of thumb.
The cost of getting it wrong
An undersized unit is the classic false economy: it looks cheaper on the quote, but BestBuilders says it runs flat out and never cools the room. An oversized one short-cycles and wastes money. Right-sized is the aim.
How we size it
At the free survey we look at the room's dimensions, glazing and orientation, insulation and how you use the space, then spec the unit that holds temperature efficiently. It's ten minutes that saves you money every summer the system runs — and it's part of the fixed quote, not an add-on.
“An undersized unit fails on exactly the days you bought it for. We size to the glass and the aspect, not the floor plan.”
Get it sized properly — free survey →
Sources
- Daikin UK — Air conditioner size guide (about 0.1 kW per m²; a well-insulated 20 m² bedroom about 2 kW, 2.5 kW if poorly insulated or sun-facing; a 35 m² living room about 3.5 kW; large south or west glazing often 10–20% more; accessed 19 Oct 2026)
- BestBuilders — Air conditioning cost UK 2026 (single-room split £1,500–£3,000; Daikin or Mitsubishi single-room unit plus install £1,800–£3,200; brand choice can move a single-room price by £400–£800; two-room multi-split £2,800–£4,500; three-to-four-room £3,500–£7,000; pipe runs over about 5 m add £150–£400; budget £70–£150 a year for a service; an undersized unit "runs flat out and never cools the room" while an oversized one "short-cycles and wastes money"; accessed 19 Oct 2026)
Datum's engineers install and service boilers, air conditioning, heat pumps and plumbing across South Hertfordshire and London. Guides are written from real jobs, not brochures.