The following table is an approximate idea of what Air-Conditioner size you need after you Calculate Length X Width:
Ex. A Room is 6mtr long and 3 mt wide= 18 Sq. Mt., So If it is a bedroom that you need either heating or Cooling you will go for a 9,000Btu/hr ( 2.5Kw)
In the same example where you have possible a lot of cold or heat coming in to a conservatory surrounded with Glass all over you will need a 18,000Btu/hr (5 Kw).
So the following table is only for a good guide what you need and how much it is going to cost you, but further we always do a detailed survey, but with the following table we found that 95% of our clients where spot on their room size calculation.
Please Note that Sizes are in BTU/hr(British Thermal Units) but now you can find them in KW(kilowatts), so the equivalent to BTU/hr to KW are as follows:
9,000 Btu = 2.5Kw 12,000Btu= 3.2Kw 18,000Btu = 5.1Kw 24,000Btu = 7Kw 30,000Btu = 8.5Kw
Please note some: Not all Air conditioning Brands are calculated the same between BTU/hr & Kw.