How to calculate the size of a split system air conditioning unit

Written by ricky gee
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How to calculate the size of a split system air conditioning unit
Mini-split air conditioners are perfect for cooling add-on rooms. (hotel room image by Albert Lozano from

Split, or mini-split, air conditioners are ductless systems that use both an outdoor air compressor and an indoor handling unit. A conduit connects the indoor and outdoor units. They are popularly used in old houses that do not have "ducted" systems already installed. Mini-split systems are also practical for room additions, that are not connected to ducts, and small apartment rooms. These conditioning systems are measured in BTUs (British Thermal Units). Determine the correct amount of BTUs that your system should have to properly cool your rooms.

Skill level:

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Things you need

  • Measuring tape

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  1. 1

    Measure the length and width of your room. Multiply its length and width together to get its square footage. For example, a room that has a length of 10 feet and a width of 10 feet has a square footage of 100 square feet.

  2. 2

    Multiply the square footage by 25 to determine the recommended BTUs of your air conditioning system. For example: 25 x 100 = 2500 BTUs.

  3. 3

    Increase the BTUs if your room is in a sunny location or has poor insulation. Add up to 50% to the total determined in step two:

    2500 + .5 x 2500 = 3750 BTUs

  4. 4

    Repeat steps 1-3 for any additional rooms. Add the totals calculated in step 3 together to get the total BTU rating for the ideal air conditioner size.

Tips and warnings

  • Add more BTUs for rooms that have poorly insulated ceilings (+4000 BTUs), lots of windows that receive significant sunshine (+1500 BTUs per window) or rooms that commonly have two or more occupants (+400 BTUs per person).

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