Key takeaways
- Start from about 100 W of cooling per square metre for an average UK room with a 2.4 m ceiling.
- Sun, big windows, a roof directly above and extra people all push the size up. North-facing rooms need less.
- 1 kW of cooling is about 3,412 BTU an hour. Most UK bedrooms land on a 2.0 or 2.5 kW unit.
- Oversizing is not safer: an oversized unit cycles on and off, dehumidifies poorly and costs more to fit.
- A sizing estimate is for shortlisting. The installer should measure the room before quoting.
Air conditioners are sold by cooling capacity, in kilowatts (kW) in the UK and British thermal units per hour (BTU/h) on many product pages. Get the size wrong in either direction and you pay for it. Too small and the unit runs flat out on the hottest afternoon and still loses. Too large and it blasts the room cold, switches off, and repeats, which wastes energy and leaves the air clammy because it never runs long enough to pull moisture out.
The starting point: watts per square metre
UK installer guidance usually starts at about 100 to 150 W of cooling per square metre of floor, for a room with a standard 2.4 m ceiling. Our calculator starts at 100 W per square metre for an average, east-facing room and then adjusts for the things that actually move a room's heat gain. If your ceiling is higher, the room holds more air, so the capacity scales up with it: a 3 m ceiling adds about a quarter.
What changes the answer
- Which way the main window faces. West-facing rooms take the hottest afternoon sun and need the most; north-facing rooms the least.
- How much glass there is. Big windows and patio doors let in far more solar heat than a wall.
- What is above you. A top-floor flat or a loft room sits under the roof, which bakes in summer.
- Insulation. Poorly insulated walls and roofs let heat in faster.
- People and equipment. Each person adds heat, as do computers, gaming PCs, televisions and cooking.
The difference is large. A 12 m² east-facing bedroom in the middle of a building comes out at about 1.3 kW, so a 2.0 kW unit. An 18 m² south-facing loft room with big roof windows comes out at about 3.0 kW, which needs a 3.5 kW unit even though it is only half as big again: about 58% more cooling for every square metre.
A quick size table
| Room size | Easy room: east-facing, middle floor | Hard room: west-facing, big windows, top floor |
|---|---|---|
| 10 m² | 1.1 kW (2.0 kW unit) | 1.6 kW (2.0 kW unit) |
| 12 m² | 1.3 kW (2.0 kW unit) | 1.9 kW (2.0 kW unit) |
| 15 m² | 1.6 kW (2.0 kW unit) | 2.4 kW (2.5 kW unit) |
| 20 m² | 2.1 kW (2.5 kW unit) | 3.2 kW (3.5 kW unit) |
| 25 m² | 2.6 kW (3.5 kW unit) | 4.0 kW (5.0 kW unit) |
| 30 m² | 3.1 kW (3.5 kW unit) | 4.8 kW (5.0 kW unit) |
| 35 m² | 3.7 kW (5.0 kW unit) | 5.5 kW (7.1 kW unit) |
Standard unit sizes, and why you round up only a little
Wall-mounted split units come in a handful of standard sizes, most commonly 2.0, 2.5, 3.5, 5.0 and 7.1 kW. Choose the smallest one that covers your estimate. Modern inverter units can run below their rated output, so a unit a size up copes fine, but jumping two sizes adds cost without adding comfort.
Cooling several rooms: multi-split sizing
A multi-split system runs several indoor units from one outdoor unit. The outdoor unit does not need to equal the sum of every room, because bedrooms and living rooms rarely peak at the same time. Installers apply a diversity allowance, and the calculator shows both the total of the rooms and a smaller estimate for the outdoor unit.
What a good installer will do
- 1
Measure each room
Floor area, ceiling height, window sizes and which way they face. - 2
Work out the heat gain
A heat-load calculation accounts for the fabric, the sun and the people, rather than a rule of thumb. - 3
Choose the unit and the position
Where the indoor unit goes affects how air spreads; where the outdoor unit goes affects noise and planning. - 4
Check the accreditation
Fitting split air conditioning needs an F-Gas certified engineer; an air-to-air heat pump for the grant needs an MCS-certified installer.
Frequently asked questions
What size air conditioner do I need for a bedroom?
Most UK bedrooms of 10 to 15 square metres need about 1.0 to 2.0 kW, so a 2.0 or 2.5 kW unit. A top-floor or loft bedroom facing south or west can need 3.5 kW.
How many BTU do I need for a room?
Multiply the kW estimate by about 3,412. A 2.5 kW room needs roughly 8,500 to 9,000 BTU per hour, and a 3.5 kW room about 12,000.
Is a bigger air conditioner better?
No. An oversized unit cycles on and off, controls humidity poorly and costs more to buy and fit. Pick the smallest standard size that covers the estimate.
Does ceiling height matter for air conditioning size?
Yes. Sizing rules assume a 2.4 m ceiling. A 3 m ceiling holds about a quarter more air, so the room needs about a quarter more capacity.
Sources
Solomon Amos, Founder & Technical Architect, GetComfort
Solomon Amos is the founder and technical architect of GetComfort, and built HomePortfolio, the property-intelligence platform behind this site's property data. He is not an installer: every accreditation and planning claim here is sourced to its primary guidance, as the methodology sets out. More about the author
