Take the floor measurements of each of the rooms that you expect to cool with the air conditioner. The height does not matter too much since the cool air sinks. Multiply the width by the length and then add them all together. This is the square footage, and since there are 337 BTUs per square foot, multiply them by 337. Write this figure down.
How to Calculate Air Conditioner Size for a House
Installing an air conditioner can make a big difference in how comfortable a home is during the hot and humid dogdays of summer. You need to make sure that you get the right size air conditioner for the size of your house, or it will run continuously without really cooling the house. Here is how you can calculate the size air conditioner unit you will need.
Things You'll Need
 Tape measure
 Calculator
Instructions


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Measure each of your south side windows and figure the square footage of each (width times height). Add the square footage up and multiply it by 871 BTUs. Do the same for the northfacing windows and multiply the square footage by 166. Add these two figures together and write it down.

3
Count how many people you expect to be living in the area to be cooled. Each person will generate about 400 BTUs each, so multiply the number of people by 400. Write this figure down.

4
Walk around and look at your lights. Write down the number of watts per light and multiply it by 4.25 BTUs. For instance, a 100 watt bulb would generate 425 BTUs. If you have 10 lights, you would have 4,250 BTUs of energy being given off. Add all the amounts together and write the sum down.

5
Check to see if you have any appliances or machines that will be running in the area to be air conditioned. See what the maximum wattage is for the appliance and then multiply it by 3.15. This is the maximum usage, and although you might not always run it at full capacity, it is better to have an air conditioner unit that can handle more rather than less than you need. Write this appliance BTUs figure down.

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Add up all the BTUs for each of the 5 Steps. This is the total BTUs you will need your air conditioner to be able to handle. When you go to shop for an air conditioner, it will need to be big enough to do at least this amount of cooling.

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Tips & Warnings
 Some of these figures can be reduced by using better insulation, shades on the windows and shade from trees.
 Photo Credit constantlychanging.com