Air conditioning in the UK is sized in kilowatts, and floor area is only the start. Use these rules of thumb to get a ballpark figure, then let a heat-load survey decide.
Fill out the form below, and a UK Aircon team member will get back to you as soon as possible.
100% Privacy Guaranteed
The right size of air conditioning unit is the one whose cooling capacity, in kilowatts (kW), matches the heat your room gains on a warm day. For a typical UK bedroom that often lands at 2.0 to 2.5kW, and for a living room 2.5 to 3.5kW. Treat those as starting points, not answers. Two rooms with identical floor areas can need very different units if one faces south with a wall of glass and the other sits on the shaded side of the house.
Below we explain how air conditioning is measured and give you a rule-of-thumb method for a ballpark figure. After that we run through the factors that move the number up or down, and why the final call should come from a proper heat-load survey.
In the UK, split air conditioners are rated by cooling capacity in kW. That's the amount of heat the unit can remove from the room, not the electricity it uses. Domestic split systems come in a fairly standard set of sizes, usually 2.0, 2.5, 3.5, 5.0 and 7.0kW, and some manufacturers offer in-between capacities.
You'll also come across British thermal units per hour (BTU/h), particularly on portable units and on websites written for the US market. One kilowatt is roughly 3,412 BTU/h, so the conversions look like this:
| Nominal capacity | Approximate BTU/h |
|---|---|
| 2.0kW | 7,000 |
| 2.5kW | 8,500 |
| 3.5kW | 12,000 |
| 5.0kW | 17,000 |
| 7.0kW | 24,000 |
Modern splits are heat pumps, so they also have a heating capacity, usually a little higher than the cooling figure. If you plan to heat with the system, that matters as well (more on this below). Capacity isn't the same as power consumption either, which means a bigger unit doesn't use proportionally more electricity. Our article on how much air conditioning costs to run explains the difference.
The quickest way to a ballpark figure is to multiply the floor area of the room in square metres (m²) by an allowance in watts per square metre (W/m²). The allowances below are a starting point only for UK homes with ceilings around 2.4m high and normal household occupancy. They don't replace a survey.
| Room situation | Rule-of-thumb allowance |
|---|---|
| Well-insulated, shaded or north-facing room | around 80 to 100 W/m² |
| Typical bedroom or living room with some sun | around 100 to 125 W/m² |
| South- or west-facing room with large windows, or a top-floor room | around 125 to 150 W/m² |
| Loft room with rooflights, or a home office full of equipment | 150 W/m² or more |
| Conservatories and glass-roofed extensions | much higher and highly variable, so always survey |
On top of the base figure, allow roughly 100W for each extra person who regularly uses the room. Then add the wattage of anything that runs while you need cooling, such as computers, monitors, gaming consoles or a large television.
4.0 × 3.5m = 14m². At 100 W/m² that is 1.4kW. Allowing a little for a laptop and lighting, a 2.0kW unit is the likely fit.
5.0 × 4.5m = 22.5m². At 125 W/m² that is about 2.8kW. Add two extra people and a TV, around 0.3kW, and you are at 3.1kW: a 3.5kW unit.
4.0 × 4.0m = 16m². At 150 W/m² that is 2.4kW. Add two screens and a PC, perhaps 0.3kW, giving 2.7kW: probably a 3.5kW unit.
Round up to the next available size, not down. Just don't jump two sizes "to be safe", because oversizing brings its own problems, as you'll see shortly.
Floor area tells you how big the room is. These factors tell you how much heat gets into it, and they're the reason a rule of thumb can be out by a whole unit size.
On a summer day, sunlight through glass is the biggest single heat source in most UK homes. A west-facing room with patio doors can gain more heat in the late afternoon than a north-facing room twice its size. It all counts: how much glass there is and which way it faces, any shade from trees or neighbouring buildings, and any blinds or solar film you have.
Solid-wall Victorian terraces behave differently from cavity-wall semis. Timber-frame new builds and flats in concrete blocks are different again. Good insulation slows heat getting in during summer. Heavy masonry soaks up heat during the day and gives it back at night, which is why some bedrooms stay warm long after sunset.
People give off heat. Two people sleeping in a bedroom add a modest load. A living room with a family of five watching television, or a home gym, adds considerably more.
Almost all the electricity used by computers, screens, servers, games consoles, fridges and older halogen lighting ends up as heat in the room. So home offices, media rooms and kitchens can need noticeably more capacity than their floor area suggests.
The W/m² allowances assume a ceiling of about 2.4m. A room with a 3m ceiling holds around 25% more air. Period properties with tall ceilings and big sash windows usually need an upward adjustment. Vaulted ceilings in extensions tend to come with rooflights as well, which adds solar gain.
Rooms directly under the roof pick up heat from the roof structure itself, on top of what comes through rooflights and dormers. Loft conversions are among the hardest rooms in a UK house to keep cool. We usually size them at the top end of the rule-of-thumb range, or above it.
| Undersized unit | Oversized unit | |
|---|---|---|
| On a hot day | Runs flat out and still cannot hold the set temperature | Reaches temperature quickly, then cycles on and off |
| Humidity | Removes moisture, but the room stays warm | Short run times remove less moisture, leaving the room cool but clammy |
| Noise | Fans and compressor often at high speed | Frequent start-ups can be noticeable |
| Efficiency | Spends long periods at full load, the least efficient operating point | Spends more time at minimum output or stopping and starting, which wastes energy |
| Wear | Long hours at high output | More start-stop cycles on the compressor |
| Purchase cost | Saves little | Pays for capacity you do not need |
Inverter compressors soften these problems by varying their output, but they can't run below a certain minimum or above their maximum. A unit sized close to the real heat load spends most of its life in the efficient middle of its range. Our guide to the wall-mounted air conditioning unit explains how inverters do this in everyday use.
A split system is an air-to-air heat pump, and plenty of people end up using it more for heating in spring, autumn and winter than for cooling in summer. If heating matters to you, the unit has to be checked against the room's heat loss on a cold day as well as its heat gain on a hot one. At UK Aircon we look at both. In a well-insulated room the cooling load usually sets the size. In a draughty room with a lot of glass, the heating requirement can be the bigger of the two. A heat pump's heating output also falls as the outdoor temperature drops, through a cold Edinburgh winter for example, and a good survey takes that into account.
An open-plan kitchen, dining and living area is sized as one space, but its shape matters. One large wall unit may not be able to throw air into the far corner of an L-shaped room or a long through-lounge. Two smaller indoor units can work better there, or a ceiling air conditioning unit in the middle.
When one outdoor unit serves several rooms, each indoor unit is sized for its own room. The outdoor unit is then chosen on the basis that rooms rarely peak at the same time. Our guide to multi-split air conditioning installation explains how that works.
Rules of thumb get you into the right range. A heat-load survey gets you the right unit. It looks at the actual glazing, orientation, construction, ceiling height, occupancy and equipment in each room, plus any need for heating. The result is a system that is neither straining nor over-specified.
The survey is free. It's also where we confirm the pipe route, the outdoor unit position and any planning considerations.
Many UK bedrooms need a 2.0kW or 2.5kW split system. As a starting point, multiply the floor area in m² by around 100 to 125 W/m², then adjust for sunny glazing, loft position, occupancy and equipment. A heat-load survey confirms the right size.
As a rule of thumb for UK homes with ceilings around 2.4m, allow around 80 to 100 W/m² for shaded, well-insulated rooms, 100 to 125 W/m² for typical rooms, and 125 to 150 W/m² or more for sunny, top-floor or loft rooms. These are starting points only. Conservatories need a survey.
No. An oversized unit cycles on and off more, removes less humidity and costs more to buy without cooling the room any better. Choose the next available size above the calculated load rather than going two sizes up.
Divide BTU per hour by about 3,412 to get kW. For example, 12,000 BTU/h is roughly 3.5kW and 9,000 BTU/h is roughly 2.6kW.
A larger unit has a higher maximum electrical input, but a correctly sized unit uses electricity in proportion to the heat it actually removes. A badly oversized unit can waste energy through frequent cycling, so correct sizing is the most efficient choice.
Planning air conditioning for one room or several? The most reliable way to get the size right is a heat-load survey by an experienced installer. UK Aircon's REFCOM-registered engineers carry out free surveys and recommend capacities room by room. We install systems from £1,800, backed by a 5-year warranty. Read about our domestic air conditioning installation service, or request your free quote and we will reply within 24 hours.
Cassettes, concealed ducted units and ceiling-suspended models keep air conditioning off the walls and spread air evenly across larger rooms. This guide helps you work out which one your space can take.
Read article : Ceiling Air Conditioning Unit: Maximising Comfort, Efficiency, and Aesthetics in Your SpaceThe wall-mounted split is the most common air conditioner in UK homes and small offices. Where it goes, how it is fitted and how it is set up decide how well it performs for the next decade.
Read article : Wall Mounted Air Conditioning Unit: Optimal Placement, Installation Steps, and Efficiency BenefitsWhat a professional air conditioning service includes, which jobs owners can do themselves, how often systems need attention through the year, and the legal checks that apply to larger installations.
Read article : Air Conditioning Maintenance: Ensuring Efficiency, Comfort, and LongevityFree, no-obligation quotes from REFCOM registered installers with over 20 years of experience. Locally owned and operated, with a 5 years warranty.
