Which size air conditioner do I need?
Answer five questions and get a capacity in BTU and kW, plus the units that match. Two minutes now saves a very annoying summer later.
A guide only. Awkward rooms, poor insulation and open-plan layouts change the answer — send us a photo and we will size it properly, free.
Four things the calculator is doing for you
Floor area is only the starting point
The old rule of 100 W per square metre works for a plain rectangular bedroom and almost nothing else. Glazing, ceiling height and what the room is used for all move the number.
Big glazing and top floors need more
A room with large glazing or one sitting directly under a warm roof can take 10–15% more capacity. It is the extra heat load in July that defeats an undersized unit.
Oversizing is not the safe option
An oversized unit reaches temperature quickly, stops, and never runs long enough to dehumidify. The room ends up cold and clammy, the compressor short-cycles, and the efficiency you paid for never materialises.
Heating needs the same care
Capacity falls as the outdoor temperature drops. A unit sized only for cooling can leave you short in February — which is why our figures assume it will be used in both directions.
Typical British rooms and what usually goes in them
These assume a 2.5 m ceiling and average glazing. Add a size for top-floor rooms or anything with large glazing or bi-fold doors.