AC size calculator: the kW and BTU each room needs
Work out the cooling capacity a UK room needs in kW and BTU/h, from its size, orientation, glazing, floor and occupants. Free, no sign-up.
Length times width. A 4 m by 3.5 m room is 14 m².
What is it used for?
Main window faces
Amount of glass
Where is the room?
Insulation
People usually in it
Computers or a TV on for hours?
Bedroom needs about
1.6kW
about 5,500 BTU/h
Unit size to shortlist
What moved the number
- Starting point: 14 m² at 100 W/m²1.4 kW
- South-facing window+15%
How this works
The calculator starts from the common UK installer rule of thumb of about 100 W of cooling per square metre for an average room with a 2.4 m ceiling, and scales it with the ceiling height.
It then adjusts for what actually changes a room's heat gain. These adjustments are GetComfort's own, deliberately moderate estimates: west-facing +20%, south-facing +15%, east +5%, north -10%; large glazing +15%, small -8%; a top floor +15%, a loft room +25%, a ground floor -5%; poor insulation +12%, good -8%; a conservatory +60%. It adds about 0.1 kW for each person beyond two, 0.6 kW for a kitchen and 0.2 kW for equipment that runs for hours.
The result is rounded up to the nearest standard wall-unit size. For several rooms, the multi-split outdoor unit is estimated at 85% of the room total, because rooms rarely need full cooling at the same time. This is a shortlisting estimate. An installer should measure the room and do a heat-load calculation before quoting.
Sources
Questions people ask
What size air conditioner do I need for a 15 m² room?
About 1.5 kW for an average, east-facing room in the middle of a building, so a 2.0 kW unit. A south or west-facing room under the roof can need 2.5 to 3.5 kW.
How do I convert kW to BTU?
Multiply by about 3,412. A 2.5 kW unit is about 8,500 BTU per hour and a 3.5 kW unit about 12,000.
Is it better to oversize an air conditioner?
No. An oversized unit switches on and off more, removes less humidity and costs more to buy. Choose the smallest standard size that covers the estimate.
Do I need a separate unit for every room?
Not necessarily. A multi-split system runs several indoor units from one outdoor unit, and the outdoor unit can be smaller than the total because rooms rarely peak together.