Air Conditioner Room Formula?

It is actually actually be quite complicated - you may be able to work it out from here:

http://www.engineeringtoolbox.com/hvac-systems-t_23.html

Guest,

Consumer Reports has an online calculator / worksheet that makes the calculation pretty straightforward, as long as you have all of the necessary information available about the space to be cooled.

(It goes a lot further than just considering the area of the room):

http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/air-conditioners/sizing-worksheet/

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Just my $0.02...

10x20x12

lengthxwidthx25 plus 1000 per window plus 400 per occupant

equals btu of unit needed