What Size Air Conditioner Do I Need? BTU Calculator & Advice

When it comes to finding the best air conditioner, you have many factors to consider. One of the most important is the air conditioner size. This doesn’t mean the actual dimensions of the unit (though that is important, too), but instead refers to the output rating of the machine.

A properly sized air conditioner will cool your room, home, or space without working too hard or removing too much humidity. This article will examine the proper ways to size your air conditioner and explain all the factors that go into sizing. We also offer a sizing calculator to make the process simple for you.

Why Correctly Sizing Air Conditioners Matter?

Correctly sizing your air conditioner is much more important than it may seem. There are many negative factors that can happen if you choose a model too small or even too large, meaning the frequent need for repairs and machines that just don’t work properly.

  • Overworking the unit.
    An air conditioner that is too small will have to work harder to reach the desired temperature. This overworking will cause part failure long before it should.
  • Improper cooling.
    If the unit you purchase is the wrong size, it won’t get accurate temperature readings of the cooling space. This can result in the room temp being much too low or too high.
  • High energy costs.
    An air conditioner that is too small for a space will run more often, cycle frequently, and consume a lot of electricity. It will be highly noticeable in your monthly energy bill.
  • Excessive humidity removal.
    If your air conditioner is too large, it will cool quickly, but it will also remove more humidity than it should. While the room will be cool, it will also be too dry, aggravating allergies or cause itching, rashes, and other dry skin conditions.

How Air Conditioners Are Rated

There are three major rating systems when talking about the size of the AC. The BTU rating is most often used for space requirements. The Energy Efficiency rating determines the cost to run the unit, and the tonnage refers to the capacity of the air conditioner.

Cooling Capacity (BTUs)

You will hear the term BTU often when looking at air conditioners. a BTU is a British Thermal Unit. Essentially it is a measurement of the amount of energy it takes to raise the temperature of 1 pound of water exactly 1 degree (Fahrenheit) at sea level.

While the energy conversion and the need to do so may be outside the scope of your concerns, what you need to know is that the higher the BTU rating, the more cooling capacity the unit will have. In short, the BTU rating directly correlates to how effective that machine is at cooling a specific square foot of space.

Energy Efficiency (SEER)
seer rating chart

When you see the SEER rating, what you are looking at is the maximum energy efficiency of that particular model. SEER stands for Seasonal Energy Efficiency Ratio and is a rating of the cooling output capabilities per watt-hour of use.

Usually, you will find the SEER on a giant yellow sticker on the air conditioner or packaging. The higher the SEER, the more efficient the machine is. However, you should remember that the SEER number is the maximum. So, it can be lower, based on other conditions in your home (poor insulation, drafts, etc.)


When dealing with central air conditioners, you will most likely shop according to the tonnage of the unit. Again, this is not physical weight but a measurement of energy. A single ton is the amount of energy (in BTUs) it takes to melt 1 ton of ice in 1 day.

Essentially 1 ton is equal to 12,000 BTUs. Using what we know from BTUs, we understand that a 1 ton unit can effectively cool a 600 square foot home. Residential ACs range from 1 to 5 tons, increasing in half-ton increments.

Air Conditioner BTU Size Chart

For the visual learners and those that just want to see how it all works together, let’s put the ratings in a handy chart. Note that the square foot coverage is a range based on all 8 climate zones of the US. ACs in colder climates have higher coverage abilities than in hotter climates.

TonnageBTUsSq. Ft. Coverage

Variables that Affect Appropriate AC Sizing

Using the BTU rating and square footage of the home is a simple way to estimate the air conditioner size. However, many other factors go into getting a better estimate.

You need to account for a lot of things besides the size of your home. Below, you will find those factors and why they are important to selecting the best-sized air conditioner.

Size of Room

Most room sizes are measured in square feet. This is the length of the room multiplied by the width. For most instances, this is the only measurement you need. However, when sizing an air conditioner, you need to consider other factors.

While the room size is important, measuring non-living spaces, not accounting for cabinets, countertops, and decorative walls can alter your measurements, making you buy an air conditioner too large for the actual room size.

Daytime Heat Gain
temperature thermostat

Ambient temperatures and daytime highs will always be a factor in the overall performance of your air conditioner. As a general rule, you should only expect your central air system to lower the temperature by 20 degrees (F). This means if it is 100 degrees outside, you should only expect to see a temperature of 80 inside.

While this isn’t written in stone, it helps to know, so you don’t think something is wrong with your HVAC system when the temperature doesn’t reach the set 72 degrees.

Daytime heat gain is the rate and overall high temperature your home experiences on average. If the day starts out at 70 degrees and reaches a high of 90, then you have a 20-degree heat gain. The placement of your condensing unit will also be affected by the heat gain. Shaded areas have a lower heat gain and are optimal for the outside unit install.

Floor Plan & Ceiling Height

A better measurement is cubic feet. This takes the square foot and multiplies it by the height of the room. Using this measurement, you can account for rooms with different ceiling heights, stairs, attic and crawl spaces, and other areas not used or in need of cooling.

The layout of your home is essential to note, too. Placing vents, running ducts, and ensuring the living areas of the house are cooled will depend on a proper set up of the system. Knowing where the doorways are, which rooms are off-angle, oddly shaped, or have non-living items (stairs, counters, etc.) in them will give you a much better and more accurate size requirement.

Insulation Level

Poorly insulated homes will lose cooling and heating much faster than adequately insulated homes. If your home lacks some fundamental insulation, such as under the house, in the attic, or even in the walls, you may need to purchase a larger capacity air conditioner.

If you also plan to add to or increase the insulation level of the home, you need to account for that when shopping for your new AC.


One often overlooked area when replacing an air conditioner is the existing ductwork. If there is ducting, it may need to be replaced. Even if it looks normal and undamaged, older ducting can split, break or have separation at the joints.

These are all areas for cold air to escape, which means the system has to work harder and run longer to cool the space with the air that does make it to the room. An HVAC technician can test your existing ducting to see if there needs to be a replacement or repairs made prior to installing the new system.

Number & Style of Windows

Windows are great for letting in sunlight and offering fresh air to circulate. However, they also are great for allowing cold air to escape and letting that sunlight heat up the room. Older windows are susceptible to drafts and may need repairs, replacement, or just weather stripping to help seal.

However, large bay or French windows can be a significant source of heat loss and can drastically change the air conditioner’s efficiency. Knowing how many windows you have, their size and style, as well as age, will help you find the perfect sized AC to counteract the heat loss while remaining efficient.

Number of People Using the Space

Surprisingly, the people in your home will also be a factor in the cooling ability of your AC. This also extends to furniture, clothing, and closets. The more items and people in a living space, the more difficult it is to cool.

Humidity is also a factor. Closets and dressers stuffed full of clothes don’t allow proper airflow and can capture and maintain humidity in that room. If you can’t keep the clothing separated and allow proper airflow, make sure the closet door or drawers shut tight enough to separate the space from the rest of the room.

Cluttered rooms full of furniture and people are the same. Preventing airflow, proper circulation, and giving off their own heat, the more items, objects, and people in a room, the more difficult it is to maintain a lower temperature.

Heat-Generating Appliances

Many of the things we use every day for comfort, quality of life, and entertainment also produce heat, which needs to be accounted for. Televisions, ovens, microwaves, and even that coffee pot add to the humidity and heat of the room so it’s not just home heaters that can raise temperatures.

You also need to factor in things like your water heater, refrigerator, and all electronic devices, even the fans, and blowers in the AC unit itself.

Calculating BTU & Required AC Size

To get an accurate and specific calculation for your home’s needs, you need to have an HVAC technician perform what is known as a Manual J calculation. This calculation uses many measurements, factors (including all those listed above), and a lot of high-level math. You can see an example of the input criteria here.

Calculating the Manual J is out of the scope of most homeowners. However, HVAC technicians have access to the high-level software that will perform all the required calculations and return the size needs based on the input. There are a few things you can do, though.

Calculate the Heat Load Requirements

If you want an educated estimate, you can calculate your heat load. This is the number (in BTUs) required by the size of your home to reach the desired temperature.The formula looks like this:

Square foot of home x Average ceiling height x desired temperature difference x sealed structure rating (average for a typical home is about 0.13).

First, you enter the square foot of your home. Let’s assume it is 2400 square feet. Next, you add in the average ceiling height. If you have mostly 8-foot ceilings, you will use 8.

Next, you must determine the average temperature outside the home compared to the desired indoor temperature. For example, if it is 90 degrees outside and you wish it to be 75 inside, the temp difference is 15.Now our formula looks like:

2400 x 8 x 15 x 0.13. This equals 37,740 BTUs.

Now you want to account for insulation, population, unused areas, etc. For an estimate, you would take between 60 and 80% of this heat load. Between 22,644 and 30,192 BTUs is going to be more accurate for your space.

Related article: Converting Kilowatts to BTU

Get a Professional Estimate

As mentioned before, the Manual J method is far more accurate and will account for everything listed in this article and more for your specific home and situation. Properly trained HVAC techs can calculate this number in just a few minutes after an inspection of your home.

For a more accurate estimate of air conditioner size needs, you can also use our AC Sizing Calculator below.

Recommendations for Sizing Different AC Types

Different air conditioner types require different calculations. When sizing for a room, space, or the entire home, you need to understand the differences.

How To Size A Central Air Conditioner
Amana AVXC20 Central Air Conditioner

Central air conditioners are generally part of an entire home climate control solution. This will include heating, ventilation, and perhaps even humidification and dehumidifying aspects. You can use the sizing calculator here on this page to get you a close estimate of what you need for your size home.

However, you also need to factor in a lot of different aspects: ceiling height, room occupancy, average daily temperatures, window size and age, and many others. However, if you are looking for a ballpark range to get an idea of what air conditioner systems or brands to shop for, you can look at two major factors: square feet and BTU output.

As a general rule (and without accounting for other factors), you need 1 BTU for every 10 square feet for cooling and 1 BTU for every 20 square feet for heating. While it may not be the most accurate method, you can get a reasonable estimate of your needs with simple math.

How To Size A Mini Split Air Conditioner

Mini-split systems use the same sizing and requirement methods as central units. The difference comes in when you factor in the type of mini split system and how many rooms are utilized.

Ducted mini split systems need to calculate square foot for each room the vents end up in as well as the overall space of the home, connected rooms, and heat pump capacity.

On the other hand, ductless mini-split systems don’t need to calculate overall size or ducting needs. They will only cool or heat a room where there is a fan unit installed. It is also wise to understand the mounting location, sun exposure, and temperatures around the heat pump.

Further, a mini split heat pump can only support a certain number of indoor units. Ensure you know your needs and expectations when sizing for a mini split system to have enough output at the heat pump for your home.

How To Size A Window Air Conditioner

When sizing a window air conditioner, you have other factors to consider. First, you don’t need the entire home’s square foot measurement. An AC window unit is not designed to cool an entire home. However, some models, especially those with oscillating vents, can cool an entire room or two.

You also need to consider installation location. If the window is in direct sunlight, the system will be less efficient as it has to battle external temperatures for cooling the condenser coils. You should also factor in other doors and windows, which can be the cause of air leaks, drafts, or other cool-inhibiting factors.

How To Size A Portable Air Conditioner

Portable air conditioners are the one AC type that relies most on square foot size over all other factors. You don’t need to worry as much about direct sunlight or even heat-producing appliances. Since you can move the unit where it performs best, you only need to worry about the capacity and window location for venting.

Still, it doesn’t hurt to figure out the size requirements for all the rooms you plan (or will be possible) for the portable model to run in. Each room needs a window for the vent hoses to exhaust the hot air. You also need to consider drain locations for the moisture collection disposal.

Tips on How to Save on AC Energy Costs

Modern air conditioners are quite efficient and work well. Many brands and models will last you several years, with central systems and mini-split ACs averaging 20+ years of reliable service.

There are, though, other things you can do to ensure the AC works at its most efficient levels and saves you money on your home energy bills.

  • Utilize fans.
    Ceiling fans and floor fans can help spread cool air around the home and individual rooms. With colder air reaching more areas of the room faster, the system won’t have to run as long or as often.
  • Keep filters clean or replaced.
    Proper airflow and circulation are crucial to the efficiency of air conditioners. Clogged or damaged filters impede performance and need to be cleaned or replaced on schedule.
  • Use window coverings.
    Windows and doors are the most significant factors for heat loss and heat exchange. Make sure your doors are sealed when closed and that windows have blinds, curtains, or dressings to help keep cool air in and hot air out.
  • Close off unused rooms and vents.
    If you have unused rooms in your home, close off AC vents in those rooms, remove room sensors or close doors. This will prevent the system from attempting to cool those rooms, resulting in higher efficiency.
  • Start cooling early.
    If you set and run your air conditioner early in the day, before the heat gets too high, the system will run better and more easily maintain the temperature inside when the outside temp rises.
  • Perform regular maintenance.
    Regular cleaning and maintenance of your AC system are crucial to its lifespan and performance. Filters, housings, coils, fans, etc. all need to be cleaned regularly. Generally, once at the start of the season before you turn it on and at the end of the season when it will no longer be running.
  • Annual inspections.
    It is always wise to hire a professional HVAC technician to inspect your entire system once a year. This will find any issues with the system, list recommended repairs, and ensure the system is properly working.
  • Repair immediately.
    When a part of the system begins to fail, you need to replace or repair it immediately. While it may not have a huge impact on the system yet, it does put higher stress on other components, and leaving it in need of repair will wear out the AC components connected to the worn part, requiring more repairs and costly replacements in the near future.

People Also Ask (FAQ)

Is a bigger air conditioner better?

In short, no. Bigger air conditioners may have been a better option 30 or more years ago. However, now with modern technologies, the energy efficiency of an air conditioner is related to more than just size. An AC that is too large can short cycle, cooling too fast and not removing any humidity. It can also run fine and remove too much humidity. You also pay much more each time the unit turns on, which will be a lot more than a properly sized unit.

How many square feet will a 3 ton AC cool?

There are too many factors to consider to give an answer based only on square footage. You need to account for climate, temperature and humidity levels, sunlight and heating source locations, and much more. However, using basic BTU conversions, you can expect a 3-ton central AC unit to cool between 1500-2100 square feet.

What other considerations are there in very hot climates?

When calculating for air conditioner size in very hot climates, you need to account more for insulation, duration of runtime (in months, as the hot seasons last longer), and heat exchange properties. Where a colder climate may not need to worry about the size of windows as much, hot climates need to account for all windows, skylights, thin interior walls, and levels of insulation.

What other considerations are there in humid climates?

In humid climates, you need to ensure the system size can handle the moisture collection. High humidity lowers the system’s efficiency rating and forces it to run longer, which can cost you more in your monthly energy bill. The best practice is to go with the size requirements based on every other factor and purchase a dehumidifier (or install a dehumidifier in your HVAC system) to keep the AC’s efficiency rating higher.

What other considerations are there in colder climates?

Colder climates actually have the biggest worries. Systems that don’t often run or for long periods are susceptible to a lot of damage. Mostly due to inactivity, rodents, pests and dirt, dust or debris build-up is common. For sizing, you will find that smaller systems are the most efficient, but you need to pay attention to vents, access points, and the refrigerant lines’ entry points, which all should be sealed when the system is not in use over the colder months to prevent damage.

Do the same methods apply for RV air conditioners?

RVs have a few different AC model types that they can utilize, though central and mini split systems are not among them. RVs are a specialized system, requiring an all-in-one unit, portable or even window AC. Sizing for RVs is done more on internal square footage along with the ducting size and routing locations, as well as the number of total intake and exhaust vents.

Do the same methods apply for sizing mobile homes?

When sizing for a mobile home, the same factors do apply. However, you also need to add more emphasis on insulation and airflow concerns. In general, mobile homes have thinner interior walls and less exterior insulation. This will increase the performance needs of the AC unit. You will often find that your measurements and requirements point you to the next half-ton to full-ton sized unit higher than what your square foot and ceiling height measurements call for.

How does one ensure peak performance for these air conditioners?

The biggest factor for a decreased performance and efficiency is lack of maintenance and cleaning. To keep your system working at its peak performance and most efficient settings, ensure the filters are changed on schedule, and the coils and fans are cleaned regularly. Annual inspections and cleanings will ensure your system performs and runs like new for years to come.


Figuring out what size air conditioner you need can be a challenge. There are a lot of factors that go into the calculations. It is critical to energy efficiency and unit performance to get the ideal size for your home, space size, climate, and budget.

This article covered all of the factors you need to consider and explained why they are important. You should have a better idea, now, on how to calculate your specific needs and will understand the HVAC tech more when they cover the Manual J calculations report with you.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.