Choosing the right size of a portable air conditioner is key to smart energy consumption and efficient cooling. So, if you are trying to determine the correct size of a portable air conditioner for your home, and you are lost with all the watts, BTUs, and square feet — we did it for you.



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To decide on the portable ac size for your home, you will need the following criteria:

  • The size of the area you need to cool down
  • Whether you need personal cooling or if you want to cool down the entire space

Unlike heating devices that are more often measured in Watts, we use BTUs (British Thermal Units) to calculate the portable air conditioner size to measure the power of cooling devices. The BTUs are units of heat, and they define how much energy you need to raise the temperature of one pound of water by 1°F.

How does it apply to cooling since we will be using the air conditioner to cool the room? Cooling devices work by removing the hot air from your space and replacing it with colder air. In this case, the BTUs measure how much heat is removed, not produced.

When you are choosing the right size for a portable air conditioner, you do not necessarily need the biggest option on the market. Moreover, wrong sizing can lead to energy waste and higher electricity bills.

How to calculate your BTU requirements for your portable air conditioner

For proper portable air conditioning sizing, you will need the square footage of the room. Don't forget about the height of the ceilings as well! We won't use them in the calculation, but the higher the ceilings, the longer it'll take to get to the desired temperature.

Most of the calculations are based on the assumption that the height of the ceilings varies from 8 to 10 feet (2.5 to 3 meters).

Other things to consider to determine the portable ac size:

  • Insulation. With high temperatures outside, it will take longer to cool down the space if it is poorly insulated and, when you turn off the cooling device, it'll warm up faster.
  • Shading. If your place is heavily shaded and hardly gets any direct light, you can easily subtract 10% from the required BTUs air conditioner when you decide on how to size a portable air conditioner for your place.
  • Natural light. On the other hand, if the place is sunny and full of natural light, add 10% to the BTU.

The magic formula for your portable air conditioner size calculator.

According to The U.S. Department of Energy, here is how you calculate an air conditioner size:

"…an air conditioner generally needs 20 BTU for each square foot of living space."

If you are looking to cool a 300ft2 room, you will need a 6000 BTU air conditioner, for example. This is a calculation based on regular sun exposure, normal insulation, and standard ceiling height.

For more detailed calculations, we invite you to check out the calculator here, where you can add more criteria, measurements, and dimensions.

Even if you know how to size a portable air conditioner, a problem can arise when, for example, you have a tiny room to cool. You can end up wasting electricity and increasing your energy bill by picking an oversized model. In reality, this can be solved by choosing a more appropriate cooling option for any occasion – personal cooling.

Evapolar — evaporative air cooler for your personal cooling.

The biggest advantage of Evapolar devices is that you can cool your personal space wherever you are. Forget the formulas, the calculation, the insulation, and the ceiling's height. If you need to feel fresh and cool right away, simply direct the airflow of the evaporative air cooler at you.

Evapolar coolers have enough capacity to cool you down during a workout, when you are sleeping, or to provide you with a fresh breeze when you are working. You can use it in all the rooms. There are no restrictions!

If you are looking for a healthy, personal and effective cooling method, Evapolar air coolers will:

  • Help you save on energy. We have calculated how much it costs to run a portable air conditioner — check it out yourself in our latest article.
  • Reach the optimal cooling in minutes. You will feel a gentle cool air right away when you position the air cooler with the airflow towards you.
  • Help you forget about sizing, measurements, and calculations. Instead, plug it in and simply let the cooling begin.

On the one hand, it is pretty simple to size an air conditioner: you take the formula and then adjust it according to the insulation, sunny days, humidity, ceilings height, and your personal preferences. Read more: Swamp Cooler Is Not Blowing Cold Air – Help!

On the other, it is easy to go wrong if there are too many criteria involved. If you need to cool down your immediate personal space, you can save money, trouble and stop the math — go for an evaporative air cooler. By the way, one doesn't exclude the other, and we think that swamp coolers and air conditioners are a perfect match in the same household!


1. What size portable air conditioner do you need for a 400ft2 room?

To calculate your portable AC unit size for a 400ft2 room (approximately 18m2), you will need to multiply 400 (the surface) times 20 (the required BTUs per square foot), which will make it an 8000 BTU air conditioner.

2. How to size a portable air conditioner for a room without windows?

Most air conditioners need a window to exhaust hot air. In this case, it is not just about the sizing a portable air conditioner but also about choosing the suitable model — it can be a ventless air conditioner or an extension to an existing HVAC system.

It can also be more expensive than a model with an exhaust pipe, which is impossible to use in a windowless space. In this case, we recommend you look at portable air coolers to cool down your personal space wherever you are, whether it is a room with windows or without them.

3. What size portable AC do you need for a bedroom?

It is true that during sleep, our bodies recover better in a chilled environment, so cooling for the place where you rest is crucial. You will need to use the same formula for any other room, where you multiply the square feet by 20BTUs/foot.

However, using an air conditioner when you are sleeping has its disadvantages. It is not uncommon to catch a cold from a/c when you are sleeping.

If you want to keep your beauty sleep fresh and safe, an evaporative air cooler will not only cool down your personal space but also keep the air humidified, which is an excellent bonus for your skin, sinuses, and hair!

4. What is the difference between a portable air conditioner and an evaporative air cooler?

The difference is in the way of cooling and the space covered. Air conditioners use the air from the outside to turn it into cool air inside, while the evaporative air coolers use evaporation. For example, in Evapolar air coolers, the air goes through special cooling pads with a basalt membrane, making it the most natural, gentle way of cooling.

5. What is the smallest air conditioner available on the market today?

While the sizes can vary, the lowest BTU power usually starts from 3000 BTUs. If you are limited by space, air coolers by Evapolar only weigh a few pounds and are ultraportable and compact. Read also: Best Quietest Portable Air Conditioners In 2022