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.
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
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
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.
Next, we’ll break this room into smaller pieces and calculate the square footage of each of those pieces.
We’ll calculate area “A” first. We know both walls are 15 feet each, so calculating this section is easy.
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:
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:
6 ft x 7.5 ft = 45 square feet
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
(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.
If your room or space is sunny during the day, increase your BTUs by 10 percent.
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.
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.
- Portable Air Conditioners vs. Window Air Conditioners
- 5 Tips for Maintaining Your Window Air Conditioner
- Why a Heat & Cool Window A/C Unit Makes a Good Investment
- Cooling Capacity Calculator: For Tips for Finding the Right BTU for You