How To Size Central Air Conditioning For Your Home Central Air Conditioning systems are sized in terms of "tons." One ton equals 12,000 BTU(British Thermal Units). Therefore, if you need a three ton system, you need 36,000 BTUs. To get the proper size system for your house, you will need to measure your home's air space. Air space, or air volume, is determined by the room length, multiplied by the width, multiplied by the height. Measure each room accurately, including closet space, hallways, bathrooms, kitchen, bedrooms, etc. After you have calculated each area volume(LxWxH), you must multiply each area by a factor of either 3, if there is no cooled area above, such as an attic, or by a factor of 2.7, if there is a cooled area above, such as the second floor of a colonial home. For instance, let's assume you have a ranch style house with a full, unfinished attic. One bedroom has an area of 9 x 12 x 8; therefore, after multiplying, the total is 864. Now, multiply by a factor of 3(no cooled area above). The total BTU required for this room is 2,592. Follow this formula for each area of your house, then total all the areas and you will have the total BTU, or cooling load, required for your house. (If you have a sloped ceiling in certain rooms, average out the ceiling height: 8 ft on one side slopes up to 12 ft on the other side, therefore, the average ceiling height would be 10 ft). Let's assume that you've added all your room areas and it total to 38,492. Since central air systems are sized by ton, with half ton increments, you are sized for a 3.5 ton system. A three ton system (3x12,000BTU) is only 36,000. Since your house is above three ton, you must go up to the next size system, which is 3.5 tons. The size system you install in your home must be measured accurately. If you install a 2 ton system in a 3 ton sized house, the home will not be cooled properly and you will use more energy since the smaller system is working harder. If you choose a larger system than necessary, say a 4 ton system (48.000BTU) in a 3 ton sized house, the system itself may not work properly and the air-handler(internal coil) may freeze due to limited air circulation. An improperly sized system may also void your manufacturer's warranty. This above formula is used by many central air professionals, however, it does not consider unusual circumstances such as: skylights, rooms with much window area,or other other conditions. Additional BTUs would be required. Website: http://www.betterlifestyles.com