Determining Ideal BTUs Per Square Foot for AC Units settings

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With scorching temps and high humidity, Florida summers only stay comfortable thanks to air conditioning. But if your air conditioning unit isn’t the right size, you run into one of two problems: either the unit it too small for your home and is running constantly to compensate, which causes energy bills to go up, or the unit is too big and you spent money on capacity you didn’t need. In both cases, you’re spending money you don’t need to. Getting the right size unit is one of the best ways to keep costs as low as possible. With that in mind, let’s look at how many BTUs per square foot your AC should have to comfortably cool your home.

What Is a BTU?

BTU stands for British Thermal Unit, and it is a measure of energy. For air conditioners, BTU measures how much energy your unit uses every hour to remove hot air from your home. Simply put, the bigger your home, the more BTUs your unit will need.

What’s the Difference Between BTUs and Tonnage?

Air conditioners often display their capacity as tonnage instead of as BTUs. Both are used to match an AC unit’s capacity with the size and cooling requirements of a home. For finding the right size air conditioner, the most important thing to know is that 12,000 BTUs are equivalent to one ton.

Factors Influencing AC Size Needs in Florida

The general rule of thumb is that you need 20 BTU per square foot of living space. However, that simple formula doesn’t hold up in all climates.

Miami is in the hottest of the climate zones. The heat and high humidity mean your AC system should be more robust than for similarly sized homes in other climates. A few other factors can also play a role in what size your unit should be, including:

  1. Ceiling height. If your home has especially high ceilings, you’ll need a bigger AC unit than homes with the same square footage but lower ceilings.

  2. Window size. Do you have large windows? Do your windows get a lot of direct sunlight and heat? Then your AC unit will need to compensate.

  3. Insulation. If your insulation is old and/or poor quality, then it cannot retain cool air as well as homes with newer, higher quality insulation. Again, this is an area where your AC unit will need to provide more power.

What Size AC Unit Do I Need?

For homes in Florida, we recommend following this formula to figure out what size AC unit you need:

  1. Start with 20 BTUs per square foot of living space.

  2. If your home has old or poor-quality insulation, add 10%. So, you are now at 22 BTU per square foot.

  3. Does your home get significant sun exposure? For most homes in Miami, the answer to that is likely “yes.” Add another 10% if so. Now, homeowners with significant sun exposure in addition to poor insulation will have 24.2 as their BTU per square foot number. Homeowners who only have poor insulation or only have significant sun exposure will use 22 BTU per square foot.

  4. Next, multiply your BTU per square foot number by your home’s square footage.

  5. Finally, divide your answer by 12,000 to find your tonnage requirements.


Let’s say you live in a 2,000 square foot home in Miami. Here’s how your numbers would break down:

  • For homeowners who have either poor insulation or significant sun exposure, you would multiply 22 by 2,000. This gives you 44,000, which is the BTU capacity you need from your air conditioner. Divide that by 12,000 and you get 3.7 tons.

  • For homeowners with both poor insulation and significant sun exposure, you would multiply 24.2 by 2,000 to get 48,400. Again, that is the BTU capacity you want your air conditioner to have. To convert this to tonnage, divide 48,400 by 12,000. That gives you a final answer of 4 tons.

The largest AC size available for homes tends to be 5 tons. If your calculations show you need more than 5 tons of cooling capacity, you will need more than one AC unit. This is especially common in larger, multi-story homes. Typically, each floor has its own AC unit.

What Is My AC’s Capacity?

To find your current AC’s tonnage, look for a sticker or other marking on the side of the condenser unit that shows the manufacturer and model number. Look for a two-digit even number between 18 and 60. Divide that number by 12 to find your tonnage.

So, for example, an AC condenser displaying the number 30 would have a tonnage of 2.5

The highest a residential AC system goes is 60, which is equivalent to 5 tons.

If you’re shopping for a new AC unit, you can search for the specific tonnage you need along with your ideal brand, or you can talk to an HVAC pro about your requirements and get a quote.

How Can I Maximize My Air Conditioner’s Efficiency?

If you already have the right size AC unit but find your home isn’t getting as cool as you want it to, there are a few immediate steps you can take.

  1. Schedule maintenance with an HVAC pro. Ideally, you’ll want to get a maintenance check on your air conditioning once a year and on your heating once a year.

  2. Consider switching to a smart thermostat. This will help you better control temperature (and can help reduce energy usage). You can get individual temperature sensors to go along with it. That way, if you find one room stays hotter than others, you can program your thermostat to cool to the correct temperature for that specific area of your home.

  3. Make sure you’re utilizing other methods of cooling your home. Ceiling fans in particular can help circulate the conditioner air more effectively.

The Bottom Line on What Size AC You Need

Properly sizing your air conditioner is crucial in a hot and humid state like Florida. Ensuring that you have the correct BTU capacity per square foot can make a significant difference in your home’s comfort and your AC’s efficiency.

Still unsure about the right size for your AC unit? Contact a local HVAC expert for a comprehensive evaluation and to ensure that your new air conditioner meets all your cooling needs efficiently.