Air con
Sizing air conditioning for a bedroom
How output is calculated, what pushes the number up, and why the biggest unit on the shelf is the wrong answer.
Written by the ARA Services engineering team · Updated August 2026
The short answer
Start with the floor area and allow roughly 100 to 150 watts of cooling per square metre. A typical London double bedroom of 12 to 16 square metres lands on a 2.5 kW wall unit, which is the smallest standard size; a hot loft room or a south-facing room with a lot of glass moves up to 3.5 kW. Oversizing backfires: the unit cools the air quickly, switches off before it has removed any humidity, and leaves the room cold and clammy while the compressor wears out.
Start with the room, not the catalogue
Measure length by width to get the floor area, then use the area as a first pass at output. Wall-mounted splits come in standard sizes — 2.5, 3.5, 5.0 and 7.1 kW — so the job is matching the room to a class rather than to a precise figure.
Up to about 15 square metres, well shaded, standard ceiling: 2.5 kW, quoted as roughly 9,000 BTU.
Fifteen to about 25 square metres, or a smaller room with heavy glazing: 3.5 kW.
Above that, or knocked-through and open-plan space: 5.0 kW and upwards, and usually a different indoor unit type.
Ceiling height above the standard 2.4 metres means you are cooling more air than the floor area suggests — add to the figure.
Tell us the room dimensions and which way the window faces, and we will tell you the size.
077 2715 4746What pushes the number up
Two rooms of the same size can need different units. These are the factors that move the calculation, and they are why a rule of thumb is a starting point rather than a specification.
Sun and glazing
South or west-facing glass, large windows, roof lights or a room over a flat roof all add heat gain through the afternoon. Add ten to twenty per cent.
Top-floor and loft rooms
Heat rises through the building and sits under the roof, and loft insulation is often thinner than it should be. Loft bedrooms almost always need the next size up.
People and equipment
Two occupants, a television, a computer or a games console are all heat sources. A home office in a bedroom changes the sum.
How the room is used
A room cooled overnight from an already hot fabric is a harder job than one kept steady all day. Inverter units handle that better than fixed-speed ones.
We survey rather than estimate, so the unit fits the room instead of the price list.
Air conditioning installationWhy bigger is not better
An oversized unit reaches the set temperature in minutes and shuts down. Short cycles like that never run long enough to condense moisture out of the air, so the room ends up cool and humid at the same time, which is the sensation people describe as clammy. The compressor also starts and stops far more often than it was designed to, which shortens its life.
An undersized unit has the opposite problem: it runs flat out on the hottest afternoons and never quite arrives. The right size runs steadily at part load, which is where an inverter system is most efficient and quietest.
Been quoted a 5 kW unit for a bedroom? Worth a second opinion before it goes on the wall.
Call 077 2715 4746The things that matter in a bedroom specifically
Noise. Look at the indoor unit’s quietest sound level rather than its headline figure, and choose a model with a proper sleep mode.
Position. The indoor unit should not blow directly onto the bed; along the wall the bed sits against usually works best.
The outdoor unit. It needs airflow, a route for the pipework and a position that will not carry noise into a neighbour’s window.
Condensate. The indoor unit produces water and it has to go somewhere sensible — gravity to a drain where possible, a small pump where not.
It is also worth remembering the unit heats. In a loft bedroom or a home office, running it in heating mode on a shoulder-season morning is usually cheaper than firing the whole central heating system for one room.
One room or a whole flat, we will size it, site it and quote it before any work starts.
Book a surveyAir conditioning questions
Do I need planning permission?
Domestic installations often fall under permitted development, but there are conditions on siting and noise, and flats, listed buildings and conservation areas are treated differently. We check the position before quoting.
Can one outdoor unit serve two bedrooms?
Yes, with a multi-split: one condenser and an indoor unit in each room, each with its own control. It is usually tidier and cheaper than two separate systems.
How noisy is it in the room?
A modern inverter unit on sleep mode is quieter than a fan on low. Most of the noise people worry about comes from the outdoor unit, which is why its position matters.
Does it need servicing?
Once a year: filters, coils, condensate and a refrigerant check. It keeps the efficiency up and the warranty intact.
Get air conditioning quoted
Tell us the rooms and we will survey, size the system and give you a fixed price.