Calculation Tool
Enter the dimensions and conditions of your room and work out the cooling capacity required. Place the air conditioning unit on a wall in the 3D room and see, as you go, how the capacity you have chosen covers the space.
Cooling capacity required
10,076 BTU/h
Recommended unit: 12,000 BTU/h
The power draw is estimated on the assumption of an A++ rated inverter unit; actual consumption will vary with the unit specified and the way it is used.
Select a capacity — the coverage shown in the 3D room updates with it.
Suitable. The unit will run at part load, efficiently and quietly.
Wall the unit is fitted to
An air conditioner of the wrong size is a problem of comfort and of running cost alike.
The unit never reaches the target temperature and runs at full output continuously. Comfort suffers, the life of the compressor is shortened and consumption is higher than expected.
The unit short cycles — the compressor reaches the target early and stops, then starts again a short while later. The evaporator coil never stays cold enough to sustain condensation, so the latent load is left in the room; the space feels cold but clammy. The inrush current drawn at every start also takes its toll on the compressor and the electrical components.
The unit runs at part load, quietly and efficiently. Humidity stays in balance and swings in temperature go unnoticed.
BASE = VOLUME (m³) × INSULATION FACTOR × CLIMATE REGION
ADDITIONS = WINDOWS (m²) × 550 + OCCUPANTS × 600 + APPLIANCES × 500 (+ KITCHEN 4,000)
RESULT = (BASE + ADDITIONS) × ASPECT FACTOR × FLOOR FACTOR
| Room area | Approximate capacity | Cooling output |
|---|---|---|
| 10 m² | 9,000 BTU/h | 2.6 kW |
| 15 m² | 9,000 BTU/h | 2.6 kW |
| 20 m² | 12,000 BTU/h | 3.5 kW |
| 25 m² | 15,000 BTU/h | 4.4 kW |
| 30 m² | 15,000 BTU/h | 4.4 kW |
| 40 m² | 21,000 BTU/h | 6.2 kW |
| 50 m² | 24,000 BTU/h | 7 kW |
The figures in the table assume a floor-to-ceiling height of 2.80 m, moderate insulation and average conditions of use. Aspect, window size, floor level and the number of occupants all alter the result appreciably; for a figure particular to your space, use the tool above with your own data.
A BTU (British Thermal Unit) is the quantity of heat required to raise the temperature of one pound of water by 1 °F. The BTU/h figure quoted for air conditioners expresses the heat the unit can remove from a space in an hour. One kW of cooling capacity corresponds to roughly 3,412 BTU/h.
The approach used in practice is to multiply the volume of the room by a factor reflecting its insulation, then add the further heat gains on top. The principal ones are the window area, the number of occupants in the space and any heat-emitting appliances. The resulting figure is rounded up to the next standard capacity available on the market and put forward as the recommendation.
The unit begins to short cycle. The compressor reaches the target temperature early and stops, then starts again a short while later. An air conditioner meets the total load in two parts: the sensible load, which lowers the temperature of the air, and the latent load, which takes out its moisture. Condensation only starts once the surface of the evaporator coil has fallen below the dew point — that is, after the first few minutes of a cycle. Short cycles seldom reach that stage, so the sensible heat ratio (SHR) is skewed, the moisture stays in the room and the space feels cold but clammy. The high inrush current drawn at every start also shortens the life of the compressor.
The indoor unit is placed on the wall from which cool air can travel the greatest distance across the room, with nothing such as a wardrobe obstructing the discharge. The mounting height is usually close to the ceiling, in the range of 2.20–2.50 m. Cool air is dense and falls; for comfort, it is important that the unit does not blow directly onto a bed or a seating area.
Yes, directly. The calculation is based on volume rather than floor area: a room of the same floor area with a ceiling at 3.50 m instead of 2.80 m calls for roughly a quarter more capacity. In spaces with high ceilings the distribution of air changes as well, so a separate assessment is needed.
This tool is intended for preliminary sizing; it is not a substitute for the detailed heat load calculation carried out as part of a project. For commercial premises, glazed-façade spaces, server rooms and areas with high ceilings, the mechanical services engineer responsible for the design should be consulted.