How to Select the Right Room Air Conditioner for Your Space

When it comes to selecting your window air conditioner, choosing the right size is the most important decision you will make. Selecting a unit too small will be too underpowered to cool your room, and you will never reach the cool temperatures you are looking for.

LG Window Air Conditioner

On the other hand, picking an oversized unit will cool the room quickly, but will inefficiently cycle on and off costing you extra money. Window air conditioners also dehumidify the air on top of cooling it. An overpowered unit will run less often leaving more humidity in the air and your room feeling muggy.

Room air conditioners receive a BTU rating, starting from around 5,000 BTUs all the way up to the most powerful around 36,000 BTUs. Once we calculate the square footage of the space we want to cool, we can decide on how powerful of a unit we need to go after. In this article, we will look at measuring and calculating our space below and then compare our calculation to the BTU conversion chart (Shown Below) to determine our BTU rating.

Calculating the Square Footage of Your Space

The first step in choosing a room air conditioner is measuring your space to calculate square feet. To get the right number you will need to do some measuring and a little geometry. So, let us cover a couple simple geometric formulas to get us started.

The formula used to calculate the square footage of a typical square or rectangular is going to be the formula used for most rooms. This formula is simple:

L x B = Square Footage

How to Measure a Square Room:

If we have a room that measures 10 feet by 12 feet our formula would be:

10ft x 12ft = 120 square feet

10ft x 12ft Room

How to Measure a Triangle-Shaped Room:

Our next formula is to calculate the area of a triangle. This will be less commonly used, but might come in handy.

L x B x ½ = Square Footage

10ft x 12ft Triangle Room

So for the triangular space pictured above our formula is:

10 x 12 x ½ = 60 square feet

How to Measure an Odd-Shaped Room:

Below we have an odd shaped room that will require a few extra calculations to get the final square footage; we will walk through those steps below:

The first step is measure each wall and map that out as we’ve done below.

Odd Shaped Room

Next, we’ll break this room into smaller pieces and calculate the square footage of each of those pieces.

Odd Shaped Room

We’ll calculate area “A” first. We know both walls are 15 feet each, so calculating this section is easy.

Odd Shaped Room - Section A

15ft X 15ft = 225 square feet

Calculating “B” will require us to use the same formula. Both walls in this space measure 4.5 feet each. So:

Odd Shaped Room - Section B

4.5ft x 4.5ft = 20.25 square feet

The formula for C is similar but requires us to half our calculation of L x B. It is also important to note that the two “legs” (non –diagonal walls) be used to calculate this formula. So in this instance:

Odd Shaped Room - Section C

6 ft x 7.5 ft = 45 square feet

Odd Shaped Room - Total Square Footage

Note: We’re using decimal numbers instead of fractions to make the calculations easier to manage. To convert from inches to decimals measure your space and keep the footage as a whole number. Take your number of inches and divide them by 12. So if you have a wall 10 feet 8 inches it would calculate to: 10 Feet + 8/12 inches (.67) = 10.67 feet

Putting it Together

Now that we have calculated the area of three pieces of our space, we’re ready to add them up to get our overall square footage of the room.

  • Total Square Footage = Area “A” + Area “B” + Area “C” = 290 Total Square Feet

Selecting the BTU Rating of Our Room Air Conditioner

Comparing our calculation to the chart below, we see that 290 sq ft comes in at the high end of a 7,000 BTU unit. You will not find a great selection of window air conditioners at the 7,000 BTU Range, but we are very close to the 8,000 BTU range as well, and moving to that BTU range is justifiable in this case.

Area To Be Cooled
(square feet)
Capacity Needed
(BTUs per hour)
100 up to 150 5,000 BTUs
150 up to 250 6,000 BTUs
250 up to 300 7,000 BTUs
300 up to 350 8,000 BTUs
350 up to 400 9,000 BTUs
400 up to 450 10,000 BTUs
450 up to 550 12,000 BTUs
550 up to 700 14,000 BTUs
700 up to 1,000 18,000 BTUs
1,000 up to 1,200 21,000 BTUs
1,200 up to 1,400 23,000 BTUs
1,400 up to 1,500 24,000 BTUs
1,500 up to 2,000 30,000 BTUs
2,000 up to 2,500 34,000 BTUs

To determine the most accurate BTU estimate, you should also consider these factors:

  • Ceiling Height
    The above estimates assume you have traditional 8-foot ceilings. If your ceilings measure higher than 8 feet, you’ll want to increase your BTU level.
  • Sunlight
    If your room or space is sunny during the day, increase your BTUs by 10 percent.
  • Shade
    If your room is shaded for most of the day, decrease your BTUs by 10 percent.
  • Number of Occupants
    If more than 2 people will occupy your room or space regularly, you should add an additional 600 BTUs of cooling power for each person.
  • Kitchen:
    If you are installing your window AC in a kitchen, increase the BTU level by 4,000.

One of the most common mistakes in selecting a room air conditioning unit is the purchase of an underpowered unit that will not properly cool the room. With the proper measurements and calculations, you will be able to determine the square footage of your room and make the wise choice for your next room AC.

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Kara Zorn

About Author

Kara Zorn is a wife and working mom who loves life hacks that help her schedule allow for as much quality family time as possible. She enjoys cooking with locally sourced food and produce and recently took up brewing Kombucha.


  1. Hasan says

    I have a question. Why we are not considering the room temperature and the temperature outside the room? Besides why don’t we consider the heat generated in the room by some electronic or electrical component?

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