What Size Air Conditioner Do I Need? A UK BTU Guide
What Size Air Conditioner Do I Need? BTU and kW Explained for UK Rooms
Buy an air conditioner that's too small and it will run flat out all afternoon without ever cooling the room. Buy one that's too big and it will switch on and off constantly, waste energy and leave the air feeling damp. Getting the size right is the most important decision you'll make, and it all comes down to BTU.
As a rough guide, a typical UK bedroom needs a 2.5kW (around 9,000 BTU) unit, a larger living room needs 3.5kW (around 12,000 BTU), and big open-plan spaces need 5kW (around 18,000 BTU) or more. Room size is the starting point, but sun exposure, ceiling height, glazing, insulation and how many people use the room all push the number up or down. A proper heat load survey is the only way to be sure.
What Is BTU?
BTU stands for British Thermal Unit. It's a measure of heat energy: one BTU is roughly the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. When you see an air conditioner rated at 12,000 BTU, that means it can remove 12,000 BTU of heat from a room every hour.
In the UK, manufacturers increasingly quote capacity in kilowatts (kW) instead. The conversion is simple: 1kW is about 3,412 BTU per hour. So a 2.5kW unit is roughly 8,500 BTU, usually sold as a "9,000 BTU" model, and a 3.5kW unit is roughly 12,000 BTU.
Air Conditioner Size Chart
Use this as a starting point for an average room with standard 2.4m ceilings and reasonable insulation:
| Unit Size | Approx. BTU | Room Size | Typical Room |
|---|---|---|---|
| 2.0kW | 7,000 BTU | Up to 15m² | Box room, small home office |
| 2.5kW | 9,000 BTU | 15 to 22m² | Double bedroom, study |
| 3.5kW | 12,000 BTU | 22 to 32m² | Master bedroom, living room |
| 5.0kW | 18,000 BTU | 32 to 45m² | Large lounge, conservatory |
| 7.0kW | 24,000 BTU | 45 to 65m² | Open-plan kitchen-diner, small office |
How to Work Out the BTU You Need
Here's a quick way to get a ballpark figure for your room:
- Measure the floor area. Multiply the length by the width in metres. A 5m × 4m bedroom is 20m².
- Allow around 100 watts per m². That 20m² room needs about 2kW, or roughly 7,000 BTU, before any adjustments.
- Add 10 to 20% for sun. South or west-facing rooms and rooms with large windows pick up a lot of solar heat.
- Add more for high ceilings, lofts and glass roofs. Loft rooms and conservatories can need 30 to 50% extra.
- Allow for people and equipment. Every extra person, PC, TV or oven adds heat. Home offices and kitchens need a bigger margin.
- Round up to the next standard size. Units come in set sizes, so pick the nearest one above your figure.
Too Small vs Too Big
Undersized Unit
- Never reaches the set temperature on hot days
- Runs at full power for hours, using more energy
- Louder, because the fan and compressor are always working hard
- Struggles to heat the room in winter too
- Extra wear shortens the system's lifespan
Oversized Unit
- Cools the air fast, then cuts out before removing humidity
- Room can feel cold and clammy at the same time
- Frequent stop-start cycling is less efficient
- Uneven temperatures and cold draughts
- You pay more upfront for capacity you don't need
Modern inverter units handle a small amount of oversizing well, because they can throttle down rather than switching fully off. That's why most engineers will size slightly up rather than down, but only by a step, not two or three.
What About Multiple Rooms?
If you want to cool or heat more than one room, you'll size each indoor unit for its own room, then choose an outdoor unit that can run them all. This is called a multi-split system. You don't simply add the room figures together, because rooms rarely need full power at the same time. An installer will calculate the right outdoor unit so you're not paying for capacity that never gets used.
Choosing between models? Our Panasonic Etherea vs TZ Super-Compact guide compares two popular units available in the sizes above. And if you're weighing up a plug-in unit, see portable air conditioner vs split system. Portable units are sold on BTU too, but they deliver noticeably less real-world cooling than their rating suggests.
Common Questions
What size air conditioner do I need for a bedroom?
Most UK double bedrooms suit a 2.5kW (9,000 BTU) unit. A large master bedroom, a loft bedroom or a room that gets full afternoon sun may need 3.5kW (12,000 BTU).
Is 9,000 BTU enough for a living room?
Only for a small living room of around 20m² or less. Most UK living rooms are better served by a 3.5kW (12,000 BTU) unit, especially with a big window or patio doors.
How many BTU per square metre do I need?
A common rule of thumb is around 340 BTU (100 watts) per m² for a standard room, rising to 450 to 500 BTU per m² for sunny rooms, loft conversions and conservatories.
Does the same size work for heating?
Usually, yes. Split systems have a heating capacity that's often slightly higher than their cooling capacity. If you plan to use it mainly for heating, mention this at survey stage. Read more in our guide to using air conditioning for heating.
Do I need a survey to choose the right size?
For a single room, the chart above gets you close. For conservatories, loft rooms, open-plan spaces or multi-room systems, a proper heat load survey avoids expensive mistakes.
Get a Free Survey
Our F-Gas qualified engineers will measure your rooms, check sun exposure and insulation, and recommend the right size of system, so you get the comfort you want without paying for capacity you don't need. We cover Dartford, Kent, Essex, Surrey and South East London.
Find out more about our air conditioning installation service.