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Information that can make your head hurt
We provide the easy calculator above to help you narrow down the results. However, we have a formula below to help you confirm that you are selecting the correct sized units if you want to get into the mathematical formula of sizing.
First measure the room or each zone you plan to heat and cool. Once you have the square footage of each zone, multiply the square footage by 25. This new number is the BTU’s needed for that zone. Below is an example of a 3 zone or 3 room system.
Room 1 measures 20 feet x 20 feet
20 feet multiplied by 20 feet gives a total square footage of 400 square feet
400 square feet multiplied by 25 gives us 10,000 BTU’s needed
Room 2 measures 15 feet x 25 feet
15 feet multiplied by 25 feet gives a total square footage of 375 square feet
375 square feet multiplied by 25 gives us 9,375 BTU’s needed
Room 3 measures 30 feet x 20 feet
30 feet multiplied by 20 feet gives a total square footage of 600 square feet
600 square feet multiplied by 25 gives us 15,000 BTU’s needed
With our scenario above the system would need to have indoor units that meet or exceed 10,000 BTU’s for room 1, 9,375 BTUs for room 2, and 15,000 BTU’s for room 3. Since these are very specific sizes we would typically recommend you to go with a 12,000 x 12,000 x 15,000 BTU system or more commonly referred to as a 12 x 12 x 15.
All systems on our site already calculate the outdoor unit size so you do not need to worry about sizing that incorrectly.
Please note that this is a general rule of thumb and different climates will require different sized units. To ensure that you are sizing the unit correctly, it is always best to contact your installer to ensure that the units you are planning on purchasing is what he or she recommends.