How to Calculate Required Air Conditioner Capacity before you buy?

Whenever we plan to buy an Air Conditioner, one common thing that comes in our mind is to choose the correct size of air conditioner for our room so as to make your AC perform effectively. After all we all want the maximum worth of our money.


At Air Conditioner Dealers we almost everywhere find the line "determine the size of your room and calculate the required BTU". The size of an air conditioner required for a given job is determined by calculating the cooling load, which is the amount of cooling that the machine will have to put out to "condition" the space. The cooling load depends upon the size and shape of the space; the number and size of windows and their orientation toward the sun; the areas of walls, ceilings, floors and the extent to which they are insulated; local climatic conditions; the wattage of electrical equipment present; and the number of people who normally occupy the space.


BTU means British Thermal Unit. The BTU per hour rating is the basic measurement in the United States for air conditioning, and it is used to specify the capacity of an air conditioner. One British thermal unit (BTU) equals the amount of heat needed to raise the temperature of 1 pound of water from 59 °F to 60 °F (15 °C to 16 °C). An air conditioner with a capacity of 12,000 BTU per hour can remove enough heat from the air.

The easiest way is to simply calculate the square feet area of your room and divide it by 600 to get the basic tonnage capacity required. Now Add 0.5 Tonnes for every 10 people occupying the room at the same time. Similarly add 0.5 tonnes for every 1500 watts of appliances or lighting present in the room (A computer would consume about 300 watts and a regular bulb, 40 to 60 watts).

Ex: A 400 Sq Ft Room with 4 people and 4 Bulbs will require Air Condition with the capacity of

Calculation will go as follows:

400/600 = 0.6667
Add: 0.2 for 4 people occupying the room
Add: 0.26 for 4 Tube or lighting present in the room

Total = 1.127… making it with round figure….An A/C with Minimum of 1.5 Tonne will be sufficient

If you buy a unit whose size is too big for your room, then it would turn on and off very frequently and will not be able to dehumidify efficiently. And also if the AC unit is too small then it would run over and over again but would fail to produce the proper cooling effect. But with all this parameters at your disposal, I hope choosing the right size of air conditioner for cooling your home or office is no longer a fuss.