Choosing the right size for a portable air conditioner is key to smart energy consumption and efficient cooling. If you're trying to determine the correct size for your home but feel lost with all the watts, BTUs, and square feet — we've done the math 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 measure the cooling capacity of air conditioners. Important: Since 2020, portable ACs are required to display a SACC (Seasonally Adjusted Cooling Capacity) rating alongside or instead of the traditional ASHRAE BTU number. SACC reflects real-world performance, accounting for heat loss through the exhaust hose and air infiltration — making it 25-40% lower than the ASHRAE rating for the same unitа. When sizing your portable AC, always check the SACC rating for accurate room coverage.
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.
CEER (Combined Energy Efficiency Ratio): Since 2025, CEER has replaced EER as the official efficiency standard for portable ACs in the U.S. and Canada. CEER measures both operating power and standby power consumption. A higher CEER means better energy efficiency and lower operating costs.
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 choosing the right size portable air conditioner, bigger isn't always better. An oversized unit will cycle on and off too frequently, failing to remove humidity effectively and wasting energy. A unit that's too small will run constantly without adequately cooling the room.
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). When using these formulas, remember that the resulting BTU is the ASHRAE rating — for portable ACs, the SACC (Seasonally Adjusted Cooling Capacity) rating is typically 25-40% lower. See our SACC vs ASHRAE guide for details.
Other things to consider to determine the portable ac size:
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Insulation. With high temperatures outside, it will take longer to cool down the space if it is poorly insulated. When you turn off the cooling device, it'll warm up faster. Poor insulation increases the cooling load, so you may need a higher BTU rating.
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Shading. If your place is heavily shaded and hardly gets any direct light, you can subtract 10% from the required BTUs.
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Natural light. If the place is sunny and full of natural light, add 10% to the BTU.
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Ceiling height above 8 ft: Add 1,000 BTUs per foot over 8 ft.
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More than 2 occupants: Add 600 BTUs per additional person.
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Kitchen: Add 4,000 BTUs if the room is a kitchen or has significant heat-generating appliances.
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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 300 ft² room, you will need a 6,000 BTU air conditioner (based on the ASHRAE rating). Important: The actual SACC rating for a portable AC with 6,000 ASHRAE BTUs would be approximately 4,000-4,500 BTU SACC. Always check the SACC rating on the Energy Guide label for accurate room coverage.
For more detailed calculations, we invite you to check out the official U.S. Department of Energy calculator, where you can add more criteria, measurements, and dimensions. You can also find additional guidance on the Energy Star website.
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. If you only need cooling within a 3-4 foot radius (a desk, bed, or workstation), Evapolar offers a low-energy alternative to traditional ACs. Evapolar devices don't require complex sizing calculations, window venting, or exhaust hoses.
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 devices are designed for personal zone cooling within a 3-4 foot radius — ideal for a desk, bed, or workstation.
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 them in any room, they require no installation, no window, and no exhaust hose. However, they work best in dry climates (humidity below 50-60%) and are designed for personal, not whole-room, cooling.
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 hand, it's 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. And one doesn't exclude the other — swamp coolers and air conditioners can be a perfect match in the same household! Use a portable AC for whole-room cooling when needed, and an Evapolar for personal cooling at your desk or bedside without running the main AC.
FAQ:
1. What size portable air conditioner do you need for a 400ft2 room?
To calculate your portable AC unit size for a 400 ft² room (approximately 18 m²), multiply 400 by 20, which gives 8,000 BTUs. Important: This is the ASHRAE BTU rating. The actual SACC rating for a portable AC with 8,000 ASHRAE BTUs would be approximately 5,000-6,000 BTU SACC. Always check the SACC rating on the Energy Guide label for accurate room coverage.
2. How to size a portable air conditioner for a room without windows?
Most portable air conditioners need a window or another vent to exhaust hot air. In a windowless room, you have a few options: a through-the-wall AC, a ductless mini-split system, or an evaporative air cooler (which doesn't require venting at all). Each option has different costs and installation requirements, through-the-wall and ductless systems are more expensive and require professional installation.
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 while sleeping can sometimes cause discomfort if the temperature is set too low or the air becomes too dry. Some people experience dry throat or nasal passages. If you prefer a more gentle cooling method, an evaporative air cooler adds moisture to the air, which can be beneficial for your skin and sinuses.
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 cooling method and the space covered. Air conditioners use refrigerants in a closed-loop system to remove heat from the air and expel it outside. Evaporative air coolers use water evaporation to cool the air, adding moisture in the process. For example, in Evapolar air coolers, air passes through special cooling pads with a basalt membrane, creating a natural, gentle cooling effect.
5. What is the smallest air conditioner available on the market today?
While sizes vary, the lowest BTU power for traditional portable ACs usually starts around 5,000-6,000 BTUs. If you are limited by space and only need personal cooling, Evapolar devices weigh as little as 750g (1.65 lbs) and are ultraportable — perfect for desktops, bedside tables, and small workspaces.
6. What does BTU SACC mean?
SACC stands for Seasonally Adjusted Cooling Capacity. It's a rating system introduced by the U.S. Department of Energy to measure a portable AC's real-world cooling performance. Unlike the older ASHRAE BTU rating, SACC accounts for heat loss through the exhaust hose and air infiltration. The SACC rating is typically 25-40% lower than ASHRAE BTU for the same unit — but it's a more accurate reflection of how the AC will perform in your home.
Read also: Best Quietest Portable Air Conditioners In 2022