What Size Air Conditioning Unit Do You Need
Choosing the right size air conditioning unit is important. A system that is too small may struggle to cool your room. One that is too powerful may use more energy than needed and provide uneven cooling.
Air conditioning capacity is usually measured in kilowatts (kW) or British Thermal Units (BTU). As a basic guide, an average UK home may need around 0.1 kW of cooling power per square metre. However, insulation, windows, ceiling height and room use can all affect this figure.
Quick Air Conditioning Size Guide
The table below provides an estimated size for rooms with standard ceiling heights and average insulation.
| Room size | Suggested aircon capacity | Approximate BTU | Typical room |
| Up to 15 m² | 2.0 kW | 7,000 BTU | Box room or small home office |
| 15–25 m² | 2.5 kW | 9,000 BTU | Bedroom or small living room |
| 25–35 m² | 3.5 kW | 12,000 BTU | Large bedroom or lounge |
| 35–50 m² | 5.0 kW | 18,000 BTU | Large lounge or kitchen-diner |
| 50–70 m² | 7.0 kW | 24,000 BTU | Large open-plan living space |
These figures are only a starting point. According to Daikin’s UK sizing guidance, typical homes may require around 0.1 kW per square metre. A professional installer should still complete a full survey before recommending a unit.
How Do I Calculate the Size of My Room?
Measure the length and width of the room in metres. Multiply these two figures to find its floor area.
For example:
5 metres long × 4 metres wide = 20 m²
Using the table above, a 20 m² room may suit a 2.5 kW air conditioning unit. This is often enough for an average bedroom, office or compact lounge.
However, floor area does not tell the whole story. A 20 m² south-facing conservatory will usually need more cooling than a shaded bedroom of the same size.
What Size Air Conditioning Unit Do I Need for a Bedroom?
A small or average bedroom will often need a unit with a capacity of between 2.0 and 2.5 kW. This should provide enough cooling for a room measuring around 9 to 25 m².
A quiet wall-mounted unit is often a good choice for bedrooms. Modern systems can maintain a steady temperature without producing strong bursts of cold air. Many can also heat the room during winter.
The unit may need more capacity if the bedroom:
- Is inside a loft conversion
- Has large or south-facing windows
- Has poor roof or wall insulation
- Receives direct sunlight for much of the day
- Has a higher-than-average ceiling
Noise levels should also be considered. Check the indoor unit’s lowest decibel rating if you plan to run it overnight.
What Size Unit Do I Need for a Living Room?
An average living room measuring 20 to 35 m² may need a 2.5 or 3.5 kW unit. Larger lounges and open-plan spaces may require a system rated at 5.0 kW or above.
Living rooms often produce more heat than bedrooms. Televisions, computers, lighting and several people can all increase the cooling demand. Large patio doors and wide windows can also allow more heat into the room.
The location of the indoor unit matters too. It should be placed where cool air can move around the room without being blocked by tall furniture.
What Air Conditioning Do I Need for Multiple Rooms?
One wall-mounted indoor unit will not usually cool several separate rooms well. Closing a door will restrict airflow, while leaving doors open may still result in uneven temperatures.
For two or more rooms, you normally have three main options.
Separate Single-Split Systems
A single-split system has one indoor unit connected to one outdoor unit. You can install a separate system in each room.
This gives you independent control of each space. However, several outdoor units may be needed, which can take up more room outside your home.
A Multi-Split Air Conditioning System
A multi-split system connects several indoor units to one outdoor unit. Each room has its own indoor unit and temperature controls.
This can be a neat option for cooling bedrooms, a lounge and a home office without installing an outdoor unit for every room. Depending on the chosen system, one outdoor unit may support several indoor units. For example, some current residential systems can serve up to five rooms, while some Mitsubishi Electric systems support up to six indoor units.
The outdoor unit must be sized to support the required indoor units. An installer will calculate the total cooling demand and consider whether every room is likely to use air conditioning at the same time.
Ducted Air Conditioning
Ducted air conditioning can cool several rooms through hidden ducts and ceiling vents. It provides a discreet finish because most of the equipment is concealed.
This option is often easier to include during a renovation, loft conversion or new build. The property must have enough space for the ducts, indoor equipment and access panels.
Can One Air Conditioning Unit Cool an Open-Plan Area?
One unit may cool an open-plan kitchen, dining room and lounge if air can move freely throughout the space. The unit must have enough capacity for the full area.
Kitchens create extra heat from ovens, hobs and other appliances. Large roof lights and bi-fold doors can also cause the room to heat up quickly. As a result, an open-plan space may need a larger system than its floor area alone suggests.
Very large or unusually shaped rooms may benefit from two indoor units. This can spread the conditioned air more evenly and reduce hot spots.
Can an Air Conditioning Unit Be Too Powerful?
Bigger is not always better. An oversized unit may cool the room too quickly before switching off. Frequent starting and stopping can waste energy and make it harder to maintain an even temperature.
It may also create cold draughts or leave some areas feeling less comfortable. Correct sizing helps the system operate steadily and efficiently.
Arrange a Professional Air Conditioning Survey
An online calculation can give you a rough idea of the capacity you may need. However, it cannot account for every feature of your home.
Subcooled can survey your property, calculate the cooling needs of each room and recommend a suitable system. Whether you need air conditioning for one bedroom, a large open-plan space or several rooms, we can help you choose an efficient setup that provides reliable year-round comfort.
Contact our team to arrange a home air conditioning survey and receive a tailored quote.