Measure the space you will be cooling with a tape measure. Find the length and width of the space.
Multiply the length and width together for each room.
Add each room's total together to get the total area to be cooled by an air conditioner.
Do not forget to measure large bathrooms, entryways and hallways. These spaces will also need to be cooled.
Divide the total area of the space that you are attempting to cool by 500 if it is residential and 400 if it is commercial. The result is the number of tons you will need for your air conditioner. For example, if your residential space is 2,500 square feet, dividing 2,500 by 500 equals 5 tons needed.
Type into your brower: inspect-ny.com/aircond/AirConditionerChart.htm. Hit enter on your keyboard.
Find the chart half way down the page. Refer to the number of BTUs needed for the number and size of rooms to be cooled.
Add 4,000 BTUs for each room without ceiling insulation and add 4,000 BTUs for each kitchens to be cooled to the base number of BTUs found in the chart. Also add 1,500 BTUs for each window that receives a lot of sunshine and 1,500 BTUs for each room that is directly above a kitchen or boiler if the boiler is creating heat during cooling times.
Add 600 BTUs for each person who will occupy the room and subtract 1,000 BTUs for each room that is on the shaded side of the structure.
Use the total number of BTUs and the total number of tons to determine the size of air conditioner needed. Air conditioner salespeople will ask you this information when purchasing an air conditioning unit.