Calculate Your Air Conditioning

(Taken with permission from the RyanAirConSpares website)
For a rough guide you can use this formula.

For a standard room use 150w/m2 or 50btu/ft2.
For a computer room or PABX room 190w/m2 or 60btu/ft2
Q. How do I convert British thermal unit (Btu) into Kilowatts (kW)?
A. To convert British thermal unit (Btu) into Kilowatts (kW) simply multiply the British thermal unit value by 0.000293

Btu x 0.000293 = kW

This is by far the better way of calculating your requirements as it is totally accurate.

To get the most out of your room air conditioner, you'll need to consider the size of the unit that best suits your needs.
The cooling capacity of room air conditioners is measured in British thermal units per hour, or Btu/h. To figure out the maximum cooling capacity for a room, you will first need to calculate the size of the room where the air conditioner will be placed.
To do this, simply multiply the width of the room by the length of the room.  Use the following information and formulas to help you with calculating your air conditioning requirements:
Heat Load

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
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