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How to Size an Air Conditioner


B efore you get into features and options, what type suits you best, and other details of the sort, you first have to determine what size AC you need. Whether you are looking for a room AC or a central system, without proper sizing, you won’t reap the benefits of its performance but continue to have an unbearable environment indoors when the hot season comes. To aid you in this sense, in the following, we will look into the sizing of room and central ACs to make sure you follow through right with the first step when buying an air conditioner.

Why Air Conditioner Size Matters

Yes, size does matter! But what size refers to in this case aren’t the physical proportions of the AC unit but rather the cooling capacity of the system. Thus, if you want to fit it right to a specific room or your entire house for that matter, you must first consider what optimal cooling performance it needs to provide.

Why You Should Not Undersize the AC

If you install a unit that is not on par with the capacity needed to cool a specific room or the whole house in case you opt for a central system, it won’t meet cooling demands. The system will have to overwork itself to achieve the desired temperature and maintain it. Likely, it won’t provide proper cooling comfort no matter how much it runs if the difference is high between its capacity and the area it must cover. As it runs non-stop and puts in all of its power to comply with your expectance, another issue that follows is that it will draw considerably more energy as opposed to if it were sized right.

The Problem with Oversizing the AC

A lot of first-time buyers think that bigger is better, and it’s not. In reality, if your AC is oversized, it will indeed cool the room or house, depending on the application you choose, but it will do it inefficiently. Optimal efficiency when discussing AC operation is achieved when cooling cycles are long and complete. If its rated capacity is too big, it will short-cycle and won’t ever reach the finalization of the cooling cycle. Woefully, short-cycling is dangerous for the system, putting too much strain on it. Damage will eventually occur, and when that happens, you need to repair the AC, depending on the severity and frequency of the situation needing to replace it altogether.

Step #1 – Calculate the Room’s Size

We begin our discussion by covering AC sizing when you install and use the system in a single, specific room. What you must first do is measure the space so that you can proceed to calculate its square footage. The calculation is based on a formula, and the formula differs depending on the shape of the room (square or rectangular rooms use a specific calculation, whereas with triangle-shaped spaces you must use another).

Calculations for a square-shaped room:

Use measuring tape that you place on the floor, preferably along the line of the wall, to measure the length and width of the room. When you have these two measurements, simply multiply them to each other, and you have the square footage of the space. (Formula: L x W = sq. ft.)

Calculations for a triangle-shaped room:

It’s unlikely for a room in your home to have this shape, but the formula comes in handy with off-shaped rooms, so it’s best to beware of it. Measure the length and width of the room. Multiply the values to each other, and then multiply by 1/2. (Formula: L x W x 1/2 = sq. ft.)

Calculations for an irregular-shaped room:

In this more complicated scenario, what you must do is measure all the walls in the room. Then, draw out a plan of the room according to its shape so that you can section it on paper into rectangular- or square-shaped zones and triangle-shaped zones. Using the formulas we previously provided, calculate the size of each resulting zone, and simply add the results to each other to obtain the room’s surface.

Step #2 – Selecting AC BTU According to the Room’s Size

Question: What does BTU mean?
Answer: The acronym stands for British Thermal Unit, and it is used to measure energy. Used for describing the performance of heating and air conditioning units, when it comes to AC systems, it refers to the amount of heat removed.

Now comes the easy part, matching the square footage you have obtained after your calculations to the corresponding needed BTU value. For this, use the info provided in the matrix below. In case the square footage is between room size options, make an estimation depending on what value it is closer to.

Room Size (sq. ft.) Corresponding Capacity (BTUs per hour)
100-150 sq. ft. 5,000 BTUs
150-250 sq. ft. 6,000 BTUs
250-300 sq. ft. 7,000 BTUs
300-350 sq. ft. 8,000 BTUs
350-400 BTUs 9,000 BTUs
400-450 sq. ft. 10,000 BTUs
450-550 sq. ft. 12,000 BTUs
550-700 sq. ft. 14,000 BTUs
700-1,000 sq. ft. 18,000 BTUs
1,000-1,200 sq. ft. 21,000 BTUs
1,200-1,400 sq. ft. 23,000 BTUs
1,400-1,500 sq. ft. 24,000 BTUs
1,500-2,000 sq. ft. 30,000 BTUs
2,000-2,500 sq. ft. 34,000 BTUs

Step #3 – Additional Room Air Conditioner Sizing Considerations


There is one last step you need to follow through with for accurate estimation of AC cooling power. What this final part consists of is taking into account some additional influencing factors, more precisely:

  • Placement in the kitchen: In case this is the room where you install it in, add another 4,000 BTUs to the matched value in the table above for the system to comply with cooling needs here. Demands are higher as temperatures get hotter in the kitchen because of meal preparation.
  • How many occupants the room has: If there are usually more than two people using the room simultaneously, add another 600 BTUs for each extra person.
  • The room is not insulated right: In this scenario, increase the BTU value by 15%.
  • The AC will run only during the night: In this scenario, decrease the BTU value by 30%.
  • Height to the ceiling: The estimates provided in the table above are valid for 8’ tall ceilings. In case the ceiling height is larger than this in your situation, opt for a higher BTU level.
  • If the room receives direct sunlight most of the day: In this scenario, increase the BTU value by 10%.
  • If the room is shaded most of the day: In this scenario, decrease the BTU value by 10%.

What About Central Systems? – How to Size a Central AC

If you seek to cool down the whole house rather than a single room or area, then you need to install a central air conditioning system. These products are more expensive upfront and costlier to run, but this is understandable seeing how they are designed to lower temperatures throughout all the rooms and spaces in a building.

For added control over the central AC unit and to reduce operating costs as well, opt for a model that features localized controls. Through the use of thermostats in each room of the house, you can manage temperatures as needed from one space to another.

Tonnage calculations:

Before you calculate, you need to learn the size of the house. Go through each room and measure length and width, and using those values, calculate square footage for each space. When you have calculations for all of these areas, simply add them together to learn the size of the house.

  • For regular climate areas: Multiply the house’s square footage by 30. Then, divide the resulting value by 12,000. From this last value, subtract 1, and you obtain the tonnage needed for the AC.
  • For hot climate areas: Multiply the house’s square footage by 30. Then, divide the resulting value by 12,000, and you have the required AC tonnage.

If your house isn’t insulated right, go a bit higher for the tonnage of the central AC to make sure the system will comply with your cooling capacity needs.

Bottom Line

Regardless if you opt for a window-mounted, portable, casement, or through-the-wall AC system, now you know how easy it is to size the system to the room it is going to be used for, the calculations and estimations being provided in this article.

As we have covered the topic of sizing a central AC unit that is supposed to cool the entirety of your home, you are covered even if you opt for this variant. Now that you have all the info you need to get started properly and install a system that complies with your demands, you can go ahead and choose the AC with which you will cool off this summer.

Tobey Hunter
Tobey Hunter
Tobey is the editor-in-chief at Optima Institute, his prior experience as a niche product reviewer in air quality improvement systems helping shape his path to conduct thorough research toward selecting and analyzing products so that customers are satisfied with the provided recommendations. During his carrier, Tobey also gathered technical input that makes him well versed in understanding advancements in modern systems, helping readers in turn to better understand how modern devices help improve life quality.