UK installers specify cooling in kilowatts. Overseas listings still use BTU per hour. As a round conversion, 12,000 BTU/h is about 3.5 kW, 9,000 BTU/h about 2.6 kW, 18,000 BTU/h about 5.3 kW and 24,000 BTU/h about 7 kW. A 12k air conditioner is therefore a capacity label, not a guarantee for a 12 square metre room. Glass, orientation, occupancy and insulation change the load. Use the conversion to read foreign ads, then size from a UK heat-gain survey.
Convert BTU labels to UK kW
A 12k, 18,000 BTU or 24,000 BTU cassette is the same conversation as a 3.5 kW, 5 kW or 7 kW split. One refrigeration ton is also about 3.5 kW, which is why 1-ton and 12k ads look similar.
Nameplate BTU is often a nominal figure at a stated test condition. UK rooms in a heatwave with west glass can need more than the round number on a web listing.
- 9,000 BTU/h is roughly 2.6 kW.
- 12,000 BTU/h is roughly 3.5 kW.
- 18,000 BTU/h is roughly 5.3 kW.
- 24,000 BTU/h is roughly 7.0 kW.
Why floor area rules fail
A shaded north bedroom and a south conservatory of the same area are not the same load. Occupancy, lighting, kitchens and roof rooms move the number more than a BTU chart.
If a quote only says 12k because the room is 12 square metres, ask for a heat-gain basis. The sizing calculator on this site is a planning aid, not a substitute for that survey.
Cassette and ducted BTU ads
24,000 BTU cassette listings are often commercial indoor units. They still need an outdoor, a void and a drain. Do not mix a supply-only BTU price with an installed UK cassette job.
Daikin 18000 BTU and Panasonic 12000 BTU searches should be translated into UK model references and kW at stated temperatures.
How to brief an installer
Give room photos, glazing and how the space is used. If you already bought a 12k idea from a video, say so. The installer may still select 2.5 kW or 5 kW after measuring.
Read the 5 kW and 2-ton guides if those labels are on the ad you were comparing.
Frequently asked questions
How many kW is a 12k air conditioner?
About 3.5 kW of cooling at the rating condition. Whether that is enough for your room depends on heat gain, not on the 12k label.
Is 1 ton the same as 12,000 BTU?
Approximately yes. Both point at about 3.5 kW. UK quotes should still be in kW for the actual models.
Should I buy the next size up in BTU?
Not as a rule. Oversizing can be noisier and control poorly. Survey beats a safety margin guessed from an advert.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.