What Size Air Conditioner Do I Need? UK kW & BTU Guide (2026)
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Air conditioning is sold in kW (and BTU, 1kW ≈ 3,400 BTU). Too small and it runs flat out and never quite cools; too big and it short-cycles, wastes energy and costs more. Use the table below as a starting point.
Rough guide by room size
| Room | Approx. size | Unit |
|---|---|---|
| Bedroom / study / small room | up to ~20 m² | 2.5kW (9,000 BTU) |
| Average living room | ~20 to 35 m² | 3.5kW (12,000 BTU) |
| Large living room / open-plan | ~35 to 50 m² | 5.0kW (18,000 BTU) |
| Very large / commercial space | 50 m²+ | 7.0kW+ or multi-split |
Things that push the size up
- Lots of glazing or south-facing sun. Conservatories and big windows gain a lot of heat.
- High ceilings. More air volume.
- Heat-producing kit. Servers, kitchens, lots of people.
- Top-floor / poorly insulated rooms.
One room or several?
For a single room, a single-split (one indoor + one outdoor unit) is simplest and cheapest. For multiple rooms, a multi-split runs several indoor units from one outdoor unit, tidier and often better value than several separate systems. See single and multi-split systems.
Don't oversize
Bigger isn't better: an oversized unit cools the air fast but short-cycles, leaving the room clammy and the bills higher. A correctly sized inverter unit runs steadily and dehumidifies properly. If in doubt, our team or the System Finder will confirm the right size for your room.
Frequently asked questions
What size air conditioner for a bedroom? A 2.5kW (9,000 BTU) unit suits most bedrooms up to around 20m².
What size for a living room? Around 3.5kW (12,000 BTU) for an average living room; 5.0kW (18,000 BTU) for large or open-plan spaces.
How do kW and BTU relate? Roughly 1kW ≈ 3,400 BTU. A 2.5kW unit is about 9,000 BTU.