What size air conditioner do I need?
Use our calculator and room-by-room guide to find a rough size in kW and BTU.
Free, no obligation
Standard or pre-charged systems
As a rough guide, allow about 100 watts of cooling per square metre of floor, then adjust for sun, ceiling height and how the room is used. That puts most bedrooms at about 2.5 kW (9,000 BTU), living rooms at about 3.5 kW (12,000 BTU) and large open-plan spaces at 5 kW or more. An installer confirms the size during a survey.
Work out your size
A common starting point is about 100 watts of cooling for each square metre of floor, adjusted for sun, room use and ceiling height. Enter your room to see a rough size.
Typical sizes at a glance
- Bedroom or studyabout 2.5 kW9,000 BTU
- Living roomabout 3.5 kW12,000 BTU
- Large living or kitchen-dinerabout 5 kW18,000 BTU
- Open-plan or small office7 kW or two units24,000 BTU+
1 kW is roughly 3,400 BTU. An installer confirms the right size at the survey.
Free sizing toolTakes 10 secondsAir conditioner sizes by room
| Room | Floor area | Typical output |
|---|---|---|
| Small bedroom or study | Up to about 20 m2 | About 2.5 kW (9,000 BTU) |
| Double bedroom or home office | About 20 to 25 m2 | About 2.5 kW to 3.5 kW |
| Living room | About 25 to 35 m2 | About 3.5 kW (12,000 BTU) |
| Large living room or kitchen-diner | About 35 to 50 m2 | About 5 kW (18,000 BTU) |
| Open-plan space or small office | About 50 to 70 m2 | About 7 kW (24,000 BTU), or two units |
Factors that affect sizing
Sun and glazing
South-facing rooms, big windows and conservatories need more cooling.
Ceiling height
High ceilings and open stairwells add volume to cool.
Insulation
Well-insulated, newer homes need less; older, draughty rooms more.
People and equipment
Offices, kitchens and busy rooms add heat.
Loft rooms and garden rooms
Rooms under the roof or with lots of glazing heat up fastest.
Too big or too small
An undersized unit struggles on hot days; an oversized one cycles on and off and controls humidity poorly.
Three steps to your quotes
Tell us about your rooms
Home or business, how many rooms and the system type you would like.
We match local installers
Only installers who cover your postcode and fit your system type.
Compare your quotes
Weigh up prices, equipment and warranties, then choose, if you want to.
Air conditioning terms explained
Split system
An air conditioner with an indoor unit in the room and an outdoor unit outside, joined by refrigerant pipes and a cable.
kW and BTU
Units of cooling or heating output. 1 kW is about 3,412 BTU per hour. Units are sold in sizes such as 2.5 kW (9,000 BTU) and 3.5 kW (12,000 BTU).
Inverter
A compressor that speeds up and slows down to hold the temperature, rather than switching fully on and off. Quieter and more efficient.
SEER and SCOP
Seasonal efficiency ratings for cooling (SEER) and heating (SCOP). Higher numbers mean less electricity for the same comfort.
F-Gas certification
The qualification needed to install and work on equipment containing fluorinated refrigerants, which most standard systems use.
Pre-charged system
A split system sealed at the factory with a non-fluorinated refrigerant and a pre-flared pipe kit, so a typical install needs no F-Gas engineer.
Multi-split
One outdoor unit serving two or more indoor units in different rooms.
Condensate
Water removed from the air while cooling, carried outside by a drain pipe or pump.
Air-to-air heat pump
Another name for a reversible air conditioner: it moves heat from outside air into the room in winter.
Air conditioning sizes: common questions
How many BTU do I need for a bedroom?
A typical bedroom needs about 2.5 kW (9,000 BTU). Larger or very sunny bedrooms may need about 3.5 kW (12,000 BTU).
How do I convert kW to BTU?
Multiply kW by about 3,412. So 2.5 kW is about 8,500 BTU and 3.5 kW is about 12,000 BTU; units are sold in rounded sizes.
Is a bigger air conditioner better?
No. An oversized unit cycles on and off, controls humidity less well and costs more. The right size runs steadily and efficiently.
Does the size for heating differ from cooling?
Units have separate cooling and heating outputs. If heating matters most, check the heating output, especially at low outdoor temperatures.
Who confirms the right size?
The installer, during a survey, by looking at the room, its windows, insulation and use.
How do I get quotes?
Enter your postcode, tell us about your rooms and compare quotes from local installers.
General information only, not technical, legal or planning advice. Installers confirm system size, siting and any permissions for your property. Updated October 2026.
Sources: The Fluorinated Greenhouse Gases Regulations (GB) and GOV.UK guidance on F-gas; Approved Document P (electrical safety) and the Building Regulations for England and Wales; the Energy Performance of Buildings Regulations (air conditioning inspections); manufacturer installation guidance for split systems.