How to measure AC size required for a room?

[The Day Dreamer]

I am purchasing another AC for a room and was wondering that how much size of AC is required for it.

Is there is any thumb rule or formula to measure required AC size?

 

[MagnusTheBrewer]

There are a number of online calculators available which take the major variables into consideration like type of home construction, windows and, r values of insulation.

 

[Slacker]

It tells you on the box.