Manual J
The Air Conditioning Contractors of America is the industry organization responsible for developing a method for calculating residential cooling loads. The result is the Manual J. Professional contractors should use it to determine the proper size of any home air conditioner they plan to install. Manual J takes into account numerous factors including the average geographical temperature, type and amount of insulation, ceiling height, exterior construction materials and square footage.
Homeowner Calculations
Most do-it-yourselfers don't have access to Manual J, nor to the sophisticated tools available to professional installers. Still, armed with a tape measure and hand-held calculator, most people can make a relatively accurate estimate of their cooling needs. The process is simple--measure the length and width of the room to be cooled and multiple the two factors to get the square footage of the room. Repeat the process for each additional room that is to be cooled and add all of the sums together to get the total square footage that requires cooling.
BTU to Ton
A ton of cooling capacity is equivalent to the cooling effect of a ton of ice melting over a 24-hour period. However, most air conditioning units today are measured in British thermal units, so a little conversion math will be required. Twelve thousand BTUs is roughly equivalent to 1 ton of cooling capacity. A well-insulated room with 100 square feet will require a 4,000 to 6,000 (a half ton) BTU air conditioner while a 12,000 BTU (one ton) unit can cool up to 500 square feet. A 2-ton (24,000 BTU) central air conditioning unit can effectively cool a 1500 square-foot residence.
Source: eHow
