How to Determine the Ductless Air Conditioner Size you Need?

Knowing how to determine the ductless air conditioner size you need involves taking into account several variables related to the area the ductless air conditioner will serve and doing some math. It’s really quite easy. Here is a brief guide to sizing a ductless air conditioner.

Start with the Room or Area Size

The place to begin is with the fact that approximately 600 square feet of floor space can be cooled for every 12,000 Btuh of capacity. Many ductless mini split systems are 9,000 Btuh which would equate to 450 square feet. An 18,000 Btuh unit might be expected to serve a 900 square feet area, a 24,000 Btuh unit a 1,200 square feet area. This number works well in areas with a standard 8 foot ceiling, no windows and an ideal climate of about 72 degrees. That’s the starting point.

Variables that Reduce the Effective Coverage

1. Big or Southerly Windows: If the area you are planning to cool has large windows, especially those facing south, reduce the coverage by 20%. In other words, the 600 square feet served by your 12,000 Btuh unit is now just 480 square feet.

2. Ceiling Height: If the room has a 9’ ceiling, reduce the service area by 15%, a 10’ ceiling, reduce it by 20%.

3. Appliances: Not only stoves produce heat. Refrigerators, home media and other appliances warm up a room. Reduce the effective area your unit can serve by 10% per appliance or electronic system. A dishwasher will produce more heat than a toaster but not as much as a stove, so use the 10% as a rough figure.

4. People: The more people a room holds, the hotter it will naturally be. If the room or area to be cooled consistently has 2 or more people in it, reduce the coverage area by 5% per person.

5. Insulation: A poorly insulated room/house/building won’t hold the cool air as well. If you know the room is poorly insulated, reduce the coverage area by 15%.

6. External Climate: A cool climate will hold cool air better, a hot climate will make a unit work harder. For a moderate climate, reduce the effective coverage by 15%. For a warm to hot climate, reduce coverage by 20%.

Now it’s time to do some math. Add up the percentages that apply. As an example, let’s say you live in Atlanta (20%) and are planning to cool a new living room addition with a home media center (10%) in it and a south-facing bay window (20%). It’s a room that has an average of 3 people in it (15%) with an 8’ ceiling (0%) and is well-insulated (0%). Your 12,000 Btu air conditioner isn’t going to effectively cool 600 Square feet. Its effective area will be 65% less, or 210 square feet. Let’s say the room is 24 x 26, or 624 square feet.

Remember, in the example, you would be able to effectively cool about 210 square feet for every 12,000 Btuh given the specific circumstances. Since 624 is roughly 3 times 210, you would need 36,000 Btuh to effectively cool the room. If your math puts you between ductless air conditioner sizes, it is better to choose the larger capacity size to make sure you can adequately cool the area the mini split air conditioner will serve. Typical capacities include 9,000, 12,000, 18,000, 24,000, 27,000, 32,000 and 36,000 Btuh, so you’ll surely be able to find a ductless air conditioner that fits your needs quite well.


4 Responses to “How to Determine the Ductless Air Conditioner Size you Need?”
  1. Steve Duncan says:

    Would 2 ductless units with 9000 btu at 26 seer be more efficient that
    one unit with 18000 btu and 19 seer?

  2. john says:

    what size ductless would i need for a room 24 by 24 with 28 computers and 28 people?

    • Doug Renshaw says:

      My addition is a master suite with a sunroom (all windows facing south) attached. The rooms can be divided with a nana wall. The square footage of the master living space is 420 with (7) medium sized windows facing north and east. Ceiling height is an issue because its a 21.5 cathedral “A” framed ceiling. The sun room is 238 S/F with an 8.5 ceiling ht. . What mini-split heat-pump system would be best here? Dual zone or single?

  3. Johnny D. Nantz says:

    My house is approx.4,000 sf. 4 br. What size minisplit heatpump btu seen or what ever would it take giving in Washington state take to make my house comfortable 70 degrees year round??
    Thank you

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