The quick answer
Most UK rooms need 2.5kW to 3.5kW
- Up to 15m²2.5kW (9,000 BTU): most bedrooms and home offices.
- 15 to 25m²3.5kW (12,000 BTU): living rooms, large bedrooms and kitchens.
- 25m² +5.0kW to 7.0kW: open-plan spaces, conservatories and large rooms.
Calculator
Air conditioner size calculator
Enter your room size and type. We'll suggest the right size and show the fitted price.
Sizing chart
Air conditioner size chart for UK rooms
| Room size | Typical rooms | Unit size | BTU |
|---|---|---|---|
| Up to 15m² | Bedroom, home office, nursery | 2.5kW | 9,000 |
| 15 to 25m² | Living room, large bedroom, kitchen | 3.5kW | 12,000 |
| 25 to 35m² | Open-plan living, kitchen diner, conservatory | 5.0kW | 18,000 |
| 35 to 50m² | Large open-plan spaces, small offices | 7.0kW | 24,000 |
| Over 50m² | Shops, offices, very large rooms | Two units or a cassette | 24,000+ |
Based on about 100 watts of cooling per m² for a typical UK room with a standard ceiling height of 2.4m.
What changes the size
Six things that mean you need more power
Sun and windows
South or west-facing rooms and big windows can need 15 to 20% more cooling.
Conservatories
Glass roofs and walls heat up fast. Allow around 40% more than a normal room.
Kitchens
Ovens, hobs and fridges add heat, so kitchens need about 25% more.
Loft rooms
Rooms under the roof get hot in summer and usually need a size up.
People and equipment
Offices, gyms and rooms with computers or many people need extra capacity.
Insulation and ceilings
Poor insulation or high ceilings add volume to cool, so allow more.
Get it right
Why the right size matters
Air conditioners are sold by cooling power in kilowatts (kW) or British Thermal Units (BTU). Too small and it struggles on hot days. Too big and it cycles on and off, leaving the room clammy and wasting energy.
Every Airomax quote is checked by an F-Gas certified engineer, so you get the right size for your room, not the one that's easiest to sell.
Correctly sized systems
- Cool the room quickly and hold the temperature
- Remove humidity, so the air feels fresh
- Run quietly at low speed most of the time
- Use less electricity and last longer
FAQs
Air conditioner sizing questions
Still unsure? Call us on 0330 043 7869, 7 days a week.
What size air conditioner do I need for a bedroom?
Most UK bedrooms up to about 15m² need a 2.5kW (9,000 BTU) unit. A large or very sunny bedroom may need 3.5kW.
What size air conditioner do I need for a living room?
A typical living room of 15 to 25m² needs a 3.5kW (12,000 BTU) unit. Open-plan living and dining spaces of 25 to 35m² usually need 5.0kW.
How many BTU do I need per square metre?
As a rule of thumb, allow about 100 watts of cooling per m², which is roughly 340 BTU per m². Add more for sunny rooms, kitchens, conservatories and loft rooms.
What happens if my air conditioner is too small?
An undersized unit runs flat out and still can't keep the room cool on hot days. It also wears out faster and uses more energy.
Is a bigger air conditioner better?
Not always. An oversized unit cools too quickly, switches on and off more, and removes less humidity, so the room can feel clammy. Correct sizing gives the best comfort and lowest bills.
How do I convert kW to BTU?
1kW is about 3,412 BTU. So 2.5kW is about 9,000 BTU, 3.5kW about 12,000 BTU, 5.0kW about 18,000 BTU and 7.0kW about 24,000 BTU.
Get the right size, fitted at a fixed price
Prefer to talk? Call 0330 043 7869, 7 days a week.