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How to size an air conditioner

Click here to download our free Excel cooling calculator

As a quick reference guide the PCM15KEH-410 aircon unit will roughly cool a 45 square metre or 480 square feet room.

The PAC L10 air con unit will roughly cool 27 square metres or 290 square feet.

These figures are based on a bright, sunny, south/west facing room.

For a more exact calculation please follow the following formula.

How much heat do you want to remove?

The above cooling capacities are given in BTU's and kW's. 1kW is equal to 3412BTUs.

There are numerous different heat sources to take into account. By adding them together you will get a feel for the amount of cooling that you require.

You need to know

  • The floor area of the room

  • The size and position of windows, and whether they have blinds or shades

  • The number of room occupants (if any)

  • The heat generated by equipment

  • The heat generated by lighting

Floor Area of Room

The amount of cooling required depends on the area of the room. To calculate the area in square metres:

Room Area BTU = Length (m) x Width (m) x 337

Window Size and Position

If, your room has no windows, you can ignore this part of the calculation. If, however there are windows you need to take the size and orientation into account.

South Window BTU = South Facing window Length (m) x Width (m) x 870

North Window BTU = North Facing windows Length (m) x Width (m) x 165

If there are no blinds on the windows multiply the result(s) by 1.5.

Add together all the BTUs for the windows.

Windows BTU = South Window(s) BTU + North Window(s) BTU


You will have to take that into account people who normally working in the space. The heat output is around 400 BTU per person.

Total Occupant BTU = Number of occupants x 400


Clearly most heat in a room is generated by the equipment. This is trickier to calculate that you might think. The wattage on equipment is the maximum power consumption rating, the actual power consumed may be less. However it is probably safer to overestimate the wattage than underestimate it.

Equipment BTU = Total wattage for all equipment x 3.5


Take the total wattage of the lighting and multiply by 4.25.

Lighting BTU = Total wattage for all lighting x 4.25

Total Cooling Required

Add all the BTUs together.

Total Heat Load = Room Area BTU + Windows BTU + Total Occupant BTU + Equipment BTU + Lighting BTU

This is the amount of cooling required so you need one or more air conditioning units to handle that amount of heat.


Disclaimer: These calculations are intended as rough guide only. Complete accuracy cannot be guaranteed