When in the market to purchase an air conditioner, the first question that comes to mind is – What size air conditioner do I need?

Whether you plan on upgrading your existing air conditioner or buying a new one, it goes without saying that it’s a significant investment you don’t plan to go wrong on.

Buying the wrong air conditioner unit can lead to unwanted expensive issues much earlier than anticipated, leading you to regret your purchase.

In this brief guide, we will try to elaborately answer the common question most homeowners have when purchasing an AC unit – what size air conditioner do I need?

How to Size Your Air Conditioner

The size of an air conditioner is measured by its cooling capacity, which is ideally British Thermal Units (BTUs) or British Thermal Units per Hour (BTUh).

For example, a one-ton air conditioner unit removes 12,000 BTUs of heat in an hour.

In other words, an air conditioner needs around 20 BTUs to cool an indoor living space.

Why Does AC Size Matter?

There are numerous reasons as to why you need to research before buying a new air conditioner. One of the primary reasons is that a smaller unit won’t be able to optimally cool the designated area it is meant to cool.

It will become a bigger problem during summers or hot days when the air conditioner will struggle to keep the internal environment pleasantly cool, and some pockets of the indoor area won’t be evenly cool.

In the end, the smaller air conditioner will work overtime to achieve the desired temperature and eventually wear out faster than its expected lifespan. It also leads to decreased energy efficiency and higher energy bills.

Similarly, an air conditioner unit that’s bigger than needed for the space will wear out quickly because it will turn on and off frequently.

Due to its increased capacity, it will quickly cool the space, turn off, and turn on again to maintain the desired temperature. This unsteady pace of operation puts tremendous stress on the air conditioner unit, eventually impacting its longevity.

Also, an air conditioner unit that’s larger than needed won’t be able to remove humidity from the air efficiently. It will lead to making the atmosphere colder and unpleasant and may even potentially cause mold.

What Size Air Conditioner Do I Need?

The air conditioner size you should ideally purchase depends on the square footage of the space it is meant for. However, these dynamics don’t always apply in the case of window air conditioner units.

When purchasing a central air conditioner, you’ll need the total square footage of your property. You can easily decipher that by multiplying the length of rooms by their width rather than adding the areas of each room.

Once you have the total living area of your home, multiply it by 20 to know the amount of BTUhs needed to cool the entire space.

As per the recommendations by Energy Star, the following adjustments should be made to derive the best size –

  • If the room is heavily shaded, the air conditioner capacity needed should be reduced by 10 percent.
  • If the room is very sunny, the capacity of the air conditioner required should be increased by 10 percent.
  • For kitchens, the capacity of the air conditioner should be ideally increased by 4,000 BTUs.
  • If there are at least two people in the room at any given time, the capacity should be increased by 600 BTUs per head.

Air Conditioner Capacity by Home Size

Home SizeAC Size (BTUs)AC Size (Tons)
600 to 1,000 square feet18,0001.5
1,000 to 1,200 square feet21,0001.75
1,300 to 1,400 square feet23,0001.92
1,500 to 1,600 square feet24,0002
1,700 to 1,800 square feet27,0002.25
1,900 to 2,000 square feet30,0002.5
2,100 to 2,300 square feet33,0002.75

Window AC Sizing by Room Type

Generally, the cooling capacity of most window air conditioners revolves anywhere between 5,000 BTUs and 12,500 BTUs.

Calculate the size of the room through the method discussed above to determine the size of the window air conditioner unit you should buy.

Also, don’t forget to include the area of any adjoining rooms without doors, as the window AC unit will be cooling those rooms as well.

For your reference, here is how many BTUs are needed to cool the rooms of different sizes –

  • Small Rooms of 150-250 Square Feet – 5,000 to 6,000 BTUs
  • Medium Rooms of 250 to 350 Square Feet – 7,000 to 9,000 BTUs
  • Large Rooms of 350 to 600 Square Feet – 10,000 to 12,500 BTUs

The thumb rule you should remember when answering what size air conditioner you need is that 20 BTUs are required for every square foot of space. This will give you a good idea of the cooling capacity you should look for.

If not, you’ll end up needing frequent AC repair service Dothan AL, unnecessarily adding to your expense.

Factors to Consider When Buying an Air Conditioner

While the primary factor to consider when purchasing an air conditioner is the total square footage of your residence, there are some other factors you must take into account.

It plays a vital role in ensuring the air conditioner’s performance and energy efficiency aren’t impacted negatively.

These factors include –

  • Climate – Buy an AC unit with a higher SEER rating for warmer climates.
  • Home Exterior – The type of material on your home’s exterior decides how much heat is reflected or absorbed, influencing the cooling efficiency required by the air conditioner.
  • Sun Exposure – South, South-West, and West-facing homes receive direct sun exposure during the summers, which may benefit from AC units with 10 to 20 percent higher capacity.
  • Insulation – Proper insulation helps keep cool air in and heat out during summers and vice versa in winters. Better insulation means improved performance and efficiency of air conditioners.
  • Windows – How many windows you’ve in your home and whether single, double, or triple-pane can make a massive difference in the AC unit’s efficiency in cooling the space. Single-pane windows are less energy-efficient, requiring higher AC units with higher tonnage.
  • Ceiling Height – If your home has higher ceilings than the standard 9-foot ceilings, add 2-3 BTUs more than recommended when calculating the cooling capacity required for your living space. (The standard is 20 BTUs per square foot)
  • Heat Generating Appliances – In kitchen and laundry rooms where the appliances generate heat, higher BTUs than the recommended 20 BTUs per square foot will be required for optimal cooling.


We hope this brief guide helps you choose the right-sized air conditioner for your residence. Not buying the right air conditioner can lead to added and unwanted expenses as well as inefficient cooling, decreased AC unit lifespan, higher energy bills, and an overall unpleasant and uncomfortable home.

If you’re still in doubt about what size air conditioner do I need – trust our experts at Long Total Comfort to help you.

As a locally owned and operated HVAC company in Dothan, AL, we understand the needs of customers in our community well and offer professional HVAC services you can rely on.

For more information or to schedule a free consultation, call us at 334-759-5170.