6000 btu's (lol)
actually, square footage is only one of the factors. the manual j will take into consideration heat gain due to leaks, insulation, construction materials, windows, etc. in some cases, it's possible to use experience and rules of thumb to determine tonnage, but if there is a question, it should be calculated by someone who has the proper tools and expertise. also, the seer rating and system configuration proposed will have some bearing on the tonnage selection. for example, a 2 stage compressor with variable speed blower and humidity control will be more efficient. if it's efficiency you are concerned about, seal the house well and add extra insulation, then spend money up front on equipment for long term savings.