Short answer: As a starting point, many UK bedrooms and studies need about 2–2.5 kW of cooling, average living rooms 3.5 kW and large or sunny open-plan rooms 5–7 kW. The right size depends on glazing, orientation, insulation and how the room is used, so a room-by-room heat load check is the reliable way to size a system.
Why floor area is not enough
Two 20 m² rooms can need very different units. A north-facing bedroom with small windows gains little heat; a south-west living room with patio doors and a TV can gain twice as much. Conservatories and loft rooms gain more again.
Rough starting points
Small bedroom or study (up to about 15 m²): 2–2.5 kW. Medium bedroom or living room (15–25 m²): 2.5–3.5 kW. Large living room or kitchen-diner (25–40 m²): 3.5–5 kW. Open-plan or very sunny spaces over 40 m²: 5–7 kW or more. Use these to understand quotes, not to choose a unit.
What goes wrong when sizing is wrong
An oversized unit cools the air fast, switches off, and leaves the room clammy because it never runs long enough to remove moisture. An undersized unit runs flat out and still struggles on hot days. Inverter units have a wide operating range, which helps, but they still need to be roughly right.
Sizing for heating
If the system will also heat the home, it must be sized for heat loss on a cold day in your area. That sometimes means a larger unit than cooling alone would need. We size heat pump systems for heating first, then check cooling.
Related services
Sources
More guides
- How much does air conditioning cost to install in the UK?
- How much does air conditioning cost to run?
- Fixed split or portable air conditioning?
- Do you need planning permission for air conditioning?
- The £2,500 grant for air-to-air heat pumps explained
- F-gas rules: what homeowners need to know
- R32, R410A and R22: refrigerants explained
- Is air conditioning the same as a heat pump?
- What happens on air conditioning installation day
- Air conditioning noise and your neighbours
- Looking after your air conditioning: an owner's checklist
- Can you fit air conditioning in a listed building?
- Air conditioning in a flat: leases, freeholders and planning
- Keeping a home cool in a heatwave
- Multi-split or separate single systems?
Reviewed October 2026. Rules and prices change; check the date and ask us if anything is unclear.