There is a direct relationship between the volume of a room (measured in cubic feet) and the tonnage size of an air conditioner unit that should be used with that room. Finding the cubic feet of a room is as simple as multiplying its length, width and height. Converting that number over to the appropriate amount of tons for AC installation can then be accomplished with  what else  more simple math.
Things You'll Need
 Tape measure
 Calculator

Check to make sure the "cubic foot" measurement of your room or home is correct. Find the length, width and height of a room, and multiply those numbers together. For more than one room, find the measurements of each room and multiply the numbers individually. Add the cubic feet measurements from each room together to find the number for your entire home.

Multiply your cubic foot measurement by 1,027 to convert to British thermal units. For example, a 50cubic foot room multiplied by 1,027 equals 51,350 British thermal units.

Divide your British thermal units number by 12,000 to convert to tons. For example, a room that requires 513,500 British thermal units per hour to cool would require a 4.3 ton air conditioning system.