BTU calculator: what size air conditioner do you need?
For a typical UK room with a 2.4 m ceiling, allow about 0.1 kW of cooling per square metre, which is about 341 BTU per hour per m². So a 20 m² bedroom needs a 2.0 kW (7,000 BTU) unit and a 35 m² living room needs a 3.5 kW (12,000 BTU) unit. Add 10 to 20% for a sunny south- or west-facing room or a kitchen, and more for a taller ceiling.
Size your roomGlass rooms gain far more heat from the sun than this rule allows for. Treat this figure as the minimum and have an installer survey the room before you choose a unit.
How we worked it out
| Room | Needs | Unit | Remove |
|---|---|---|---|
| Total | |||
For two or more rooms an installer will often suggest a multi-split: one outdoor unit feeding an indoor unit in each room. They size the outdoor unit for the whole house, so give them this list when you ask for quotes.
What size room does each air conditioner cool?
The largest room each standard wall-mounted unit suits, at 0.1 kW per m² with a 2.4 m ceiling. A sunny room is one facing south or west with large windows, which needs about 20% more.
| Unit size | Sold as | Average room up to | Sunny room up to |
|---|---|---|---|
| 2.0 kW | 7,000 BTU | 20 m² | 16 m² |
| 2.5 kW | 9,000 BTU | 25 m² | 20 m² |
| 3.5 kW | 12,000 BTU | 35 m² | 29 m² |
| 4.2 kW | 15,000 BTU | 42 m² | 35 m² |
| 5.0 kW | 18,000 BTU | 50 m² | 41 m² |
| 6.0 kW | 21,000 BTU | 60 m² | 50 m² |
| 7.1 kW | 24,000 BTU | 71 m² | 59 m² |
Bigger than 71 m²? That usually means two units or a larger system, which needs a survey.
How the calculation works
- Start with 0.1 kW of cooling per square metre of floor, for a room with a 2.4 m ceiling.
- Taller ceiling: more air to cool, so the figure goes up in proportion. A 3 m ceiling adds 25%.
- Sun: add 10% for a room that gets some sun and 20% for one facing south or west with large windows.
- Kitchen: add 15% for the heat from cooking.
- People: add 0.18 kW for each person over 2 who is usually in the room.
- Loft, top-floor or older draughty room: add 15%.
- Convert to BTU at 3,412 BTU per hour for each kW, then pick the next standard unit size up.
This gets you to the right size in most rooms. An installer confirms it on the survey, because windows, walls and what's above the room all change the real heat load.
Method from Daikin UK air conditioner size guide and iHeat: what size air conditioner do I need?. Last reviewed September 2026.
BTU and sizing questions
What size air conditioner do I need for my room?
For a typical UK room with a 2.4 m ceiling, allow about 0.1 kW of cooling per square metre, which is about 341 BTU per hour per m². So a 20 m² bedroom needs a 2.0 kW (7,000 BTU) unit and a 35 m² living room needs a 3.5 kW (12,000 BTU) unit. Add 10 to 20% for a sunny south- or west-facing room or a kitchen, and more for a taller ceiling.
What size room does a 9,000 BTU air conditioner cool?
A 9,000 BTU unit is a 2.5 kW unit. It suits a room of up to about 25 m² with average sun and a 2.4 m ceiling, or about 20 m² if the room faces south or west with large windows.
What size room does a 12,000 BTU air conditioner cool?
A 12,000 BTU unit is a 3.5 kW unit. It suits a room of up to about 35 m² with average sun and a 2.4 m ceiling, or about 29 m² in a sunny south- or west-facing room. For a big open-plan space of 50 m² or so, look at an 18,000 BTU (5.0 kW) unit.
How do I convert kW to BTU?
1 kW of cooling is about 3,412 BTU per hour. The BTU figures units are sold under are rounded, so a 2.5 kW unit is sold as 9,000 BTU although the exact figure is 8,530 BTU per hour.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and switches off before it has taken much moisture out, so the room can feel clammy, and it costs more to buy. Modern inverter units can turn themselves down, so a small margin is fine, but choose the size that fits the room.
Is the same size right for heating?
Most split air conditioners heat as well as cool. If you plan to use one as a main source of heat, check the heating output at low outdoor temperatures on the spec sheet, and ask the installer to work out the room's heat loss. The cooling size on its own is not enough to decide that.
What size air conditioner does a conservatory need?
Glass rooms gain far more heat from the sun than the floor-area rule allows for, and lose it fast in winter. Treat the calculator's figure as the minimum for a conservatory or glass-roofed room, and have an installer survey it before you choose a unit.
More detail in our guide to sizing air conditioning, and typical fitted prices on the prices page.
Know your size? Get it fitted properly.
We pass your enquiry to up to three F-Gas certified local installers, who check the sizing on the survey before they quote.