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How to Size an Air Conditioner

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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 seek a room AC or a central system, you won’t reap the full benefits of its performance without properly sizing the unit. Instead, the environment indoors will continue to feel unbearable when the hot season arrives. 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 system’s cooling capacity. 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 necessary to cool a specific room or the whole house if 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 expectations, 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 founded on a formula, and the formula differs according to chamber shape (square or rectangular rooms use a specific calculation, whereas with triangle-shaped spaces you must use another).

Calculations for a square-shaped space:

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 area’s square footage. (Formula: L x W = sq. ft.)

Calculations for a triangle-shaped space:

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 room length and width. 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 space:

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 stand for?
Answer: It is an acronym 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 volume of heat removed.

Now comes the easy part, matching the square footage you have obtained to the corresponding necessary BTU value. For this, utilize 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

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There is one last step to take 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, 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 a couple of 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:

First, you need to learn the size of the house. Go through each chamber 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.

Markus Mackay
Markus Mackay
Our resident expert in all things AC-related, Markus is a never-ending source of knowledge regarding how these systems work. What makes him so savvy in this field is his direct experience that he acquired while working in sales. The connections he made during that period also came in handy here as Markus knows a lot of experts in this field. With each article uploaded, he makes sure to gather the expertise of his more experienced ex-peers as well, making sure to provide readers with reliable reviews and articles.