Sizing considerations

Factors affecting the size of the system
Sizing considerations illustrative image

Calculating the size of heat pump or air conditioner required depends on the following factors:

  • Size of the room in m3 and floor area (m2)
  • How well insulated the walls and ceiling/roof are
  • Size and type of the windows
  • Number of heat sources in the room (people / equipment such as PCs)
  • The thermal performance required - how much heating/cooling is required

The principles for cooling and heating are similar - you are changing the temperature in the room.

There are many sizing calculators on the web - some are listed below, including Checkmark’s own calculator.

BTU and kW

Systems often quote sizes in BTUs (British Thermal Units) as well ask kW (kilowatts). To convert kW to BTUs multiply by 3414.


A very general rule is a minimum of:

  • 2 kW to 2.5 kW (7000 to 8500 BTUs) for a small 18m2 room
  • 3 kW to 3.5 kW (10000 to 12000 BTUs) for a medium 24m2 room
  • 4.5 kW to 5 kW (15000 to 17000 BTUs) for a large 36m2 room.

Of course, the factors listed above all affect the system size requirement.

Obviously, an undersized unit won’t provide adequate heating/cooling and the system will run continuously trying to play ‘catch-up’. An oversized system could cycle on and off too frequently, adding strain to the unit and potentially reducing lifespan. However, most systems now have ‘invertor’ technology which allows the compressor to run at variable speeds, which helps mitigate the latter issue.

Air conditioner calculators
Typically used for mini-split systems, are likely a better option for sizing a small installation such as a home office, cabin or workshop.

Heat pump calculators
Usually used for multiple room heat pumps and hot water.

Apart from our own calculator, these links are not recommendations. All calculators differ and should only be used as an approximate guide.

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