The amount of heat generated is known as the heat gain or
heat load. Heat is measured in either British Thermal Units
(BTU) or Kilowatts (KW). 1KW is equivalent to 3412BTUs.

The heat load depends on a number of factors, by taking into
account those that apply in your circumstances and adding them
together a reasonably accurate measure of the total heat can be
calculated.

Factors include:

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

Occupants

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

Equipment

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

Lighting

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.