For many people, buying the biggest air conditioner they can afford might sound like a good idea. But bigger is not always better. A unit that is too large can cost more than necessary, while one that is too small may struggle on the hottest days.
Room size is a great first step, but it is not the whole answer. Insulation, windows, sunlight, ceiling height and connected rooms all affect how much cooling you need.
On this page
- Air conditioner room-size chart
- What do BTU and kW mean?
- What determines the size you need?
- Why your UK location matters
- What happens if the unit is too small?
- What happens if the unit is too large?
- Do you need one unit or several?
- Get a more accurate estimate
Air conditioner room-size chart
The table below provides a starting point for a typical UK home with average insulation, a normal ceiling height and no unusually large amount of glass or direct sunlight.
| Room size | Typical example | Approximate capacity |
|---|---|---|
| Up to 15 m² (161 sq ft) | Small bedroom or study | 1.5–2.0 kW / 5,000–7,000 BTU/h |
| 15–25 m² (161–269 sq ft) | Main bedroom or small living room | 2.0–3.5 kW / 7,000–12,000 BTU/h |
| 25–40 m² (269–431 sq ft) | Living room or kitchen-diner | 3.5–5.0 kW / 12,000–17,000 BTU/h |
| 40–60 m² (431–646 sq ft) | Large lounge or open-plan area | 5.0–7.0 kW / 17,000–24,000 BTU/h |
Use this chart only as a rough guide. A sunny loft bedroom may need more cooling than a larger, shaded ground-floor room. Conservatories and rooms with very large windows need a more detailed calculation.
For a sleeping room, see our bedroom air-conditioning guide for sizing and night-time considerations.
What do BTU and kW mean?
BTU/h and kW both describe how much cooling an air conditioner can provide.
Portable air conditioners typically show their size in BTU/h. Wall-mounted split systems more commonly use kW.
Some common sizes are:
- 7,000 BTU/h = about 2.0 kW
- 9,000 BTU/h = about 2.6 kW
- 12,000 BTU/h = about 3.5 kW
- 18,000 BTU/h = about 5.3 kW
- 24,000 BTU/h = about 7.0 kW
Do not confuse cooling capacity with electricity use. A 3.5 kW air conditioner does not usually need 3.5 kW of electrical power. Its real consumption depends on its efficiency and how hard it needs to work.
What determines the size you need?
Four areas have the most significant effects.
1. Room size and ceiling height
Measure the length and width of the room. Multiply them to get the floor area:
Length × width = floor area
A room of 4 metres by 5 metres has a floor area of 20 m². In imperial measurements, a room of 13 feet by 16 feet has 208 sq ft.
Do not forget the ceiling height. A room with a high or vaulted ceiling contains more air and normally needs more cooling than a room with the same floor area and a standard ceiling.
2. Insulation and glazing
Good insulation slows the heat entering your home and keeps the room cool. Poor insulation and draughts make the air conditioner work harder.
An older UK home may have improved loft insulation and modern double glazing. A newer extension may still overheat because it has a glass roof or wide patio doors.
Roof insulation matters in top-floor and loft rooms in particular. If you can improve it before installing air conditioning, you may be able to choose a smaller unit and spend less on cooling.
3. Windows, sunlight and direction
A west-facing room can become very hot in the afternoon and evening. South-facing windows can receive strong sun for much of the day.
Large windows, rooflights and patio doors also increase heat gain. Shade from trees, awnings or shutters can reduce it.
A shaded north-facing bedroom will normally need less cooling than a bright south- or west-facing room of the same size.
4. Connected rooms and how you use them
Include every space the air conditioner will try to cool. If your living room opens directly into a dining room or kitchen, measure the whole connected area. An open staircase can also let cool air escape upstairs.
People and appliances add heat. A quiet bedroom used by one person is different from a kitchen-living room where several people cook, work and watch television.
Your preferred temperature matters too. Cooling a room to 21°C during a heatwave requires more capacity than keeping it at 24°C or 25°C.
Why your UK location matters
The UK does not have one summer climate. A flat in central London may face hotter days and warmer nights than a coastal home or a house in northern Scotland.
The room itself may be more important than its postcode. A shaded bedroom in Kent might stay cooler than an uninsulated loft room in Yorkshire.
A good calculation should use realistic hot-day temperatures for your area, not only the UK’s average summer temperature. The Met Office expects UK summers to become hotter and more likely to be dry.
What happens if the unit is too small?
An undersized air conditioner may run at maximum output for long periods without reaching the temperature you selected.
In hot weather, the room may stay too warm. The unit may also become noisier, use more electricity and experience more wear.
Do not choose a smaller model simply because it costs less to buy. It may not save money if it constantly struggles to cool the room.
What happens if the unit is too large?
An oversized unit can cool the room too quickly and then switch off. It may start again soon afterwards, causing uneven temperatures, draughts and repeated short cycles. Our guide to what happens when an air conditioner is too big for a room explains why it can also leave the room feeling clammy.
Modern inverter air conditioners can lower their output when the room needs less cooling. This helps, but it does not make careful sizing unnecessary.
The aim is not to buy the biggest unit. It is to find the size that suits the room.
Do you need one unit or several?
A single wall-mounted air conditioner can cool one room or a genuinely open-plan area. It will not cool several closed bedrooms evenly. Cool air does not travel well through narrow doorways or around corners.
If you want to cool several rooms, calculate each one separately. A larger unit may suit the lounge, while smaller units suit the bedrooms. You may also save energy by cooling only the rooms you use.
Anyone installing or working on an air conditioner that contains F gas must have the correct qualifications. See the official GOV.UK guidance on F-gas qualifications.
Get a more accurate estimate
A room-size chart can point you in the right direction. It cannot see that your bedroom lies under an uninsulated roof or that your lounge leads into a sunny kitchen.
CostToCool’s Plan cooling for my home calculator uses the details that make your home different. Enter your room measurements, ceiling height, insulation and sunlight, and add any connected areas. You can use metric or imperial measurements.
You will receive a recommended capacity range in both kW and BTU/h, together with an estimate of electricity use and running cost.
Use the result as a practical starting point. Before buying a fixed system, ask a qualified installer to confirm the cooling requirement and installation design for your home.