Evaporative Cooler Sizing

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Are you ready to buy an evaporative cooler? The price is right - evaporative coolers (also called swamp coolers) can cost up to half the price of compressor-based air conditioners with similar cooling capacities. Evaporative coolers also use less energy to run - as much as 4 times less - which makes them appealing to anyone who is worried about the environment, or to those who just want to save a buck on their electric bills.

It's important, though, that you pay attention to evaporative cooler sizing when you're making your purchase. There are many choices available in the market, and choosing a unit of the wrong size will leave you feeling dissatisfied. In this short guide, we'll show you the basics of evaporative cooler sizing so you can make the right selection to meet your needs, and you won't be left feeling the heat!

Evaporative cooler sizing is measured in CFM

Unlike air conditioners, which have cooling capacities measured in British Thermal Units (BTUs), evaporative cooler sizing is based in CFM, or Cubic Feet per Minute. The CFM rating of an evaporative cooler describes how much air the cooler can circulate in the space of 60 seconds. Essentially, it's a factor of the size of the fan plus the capacity of the motor that powers it. A large, fast fan will have a significantly larger CFM than a smaller, slower fan, and therefore be capable of cooling a larger area.

The CFM of an evaporative cooler might range from 400 CFM to 20,000 CFM. So how do you know which size is right for you? The answer to your evaporative cooler sizing questions is as easy as simple arithmetic.

Evaporative cooler sizing for indoor use

Formula for Evaporative Cooler Sizing

Room Height x Room Width x Ceiling Height = Cubic Footage

Cubic Feet / 2 = Cubic Feet Per Minute (CFM)


8' x 10' x 8 ??? = 640 cubic feet

640 / 2 = 320 CFM*

*Minimum CFM needed for an evaporative cooler to work effectively in this room

The first step to evaporative cooler sizing is to know where you intend to use the cooler. If you choose the wrong size cooler for your room, it won't be able to cool the space effectively. For example, a small, portable unit won't work well in a large workshop or garage.

To start sizing evaporative coolers, you'll first need to know the cubic feet of the room you intend to cool. The cubic footage of a room is acquired by multiplying the square footage by its height. For example, an average bedroom might be 8 x 10 feet, with 8-foot high ceilings. To calculate evaporative cooling sizing, multiply 8 x 10 x 9 for a total of 640 cubic feet.

Next, to calculate the evaporative cooler sizing you'll need, divide the total cubic footage by 2. The resulting answer will tell you what CFM you need for your evaporative cooler to effectively cool your room.

Yup, it's that simple! When we apply this evaporative cooler sizing equation we can easily see that we will need an evaporative cooler of about 320 CFM in order to keep our bedroom cool through the hot, dry summer.

If you are planning on moving your evaporative cooler from room to room, then choose your unit according to the size of the largest room you intend to cool. Unlike a portable air conditioner, there is no harm in selecting an evaporative cooler with a greater capacity than the room requires.

Evaporative cooler sizing for outdoor use

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Once we start considering evaporative cooler sizing for outdoor use, calculating the needed CFM becomes a little more difficult - there's no ceiling measurement to use in our equation! So how do you determine the proper evaporative cooler sizing for your needs?

Most product listings will indicate a coverage area in square feet, which will give you an idea of how far the fan will throw the cooled air. For example, a 400 CFM swamp cooler will cover about 150 square feet, while a 22,000 CFM unit can cool down an area up to 4,400 square feet. A large blower with a low horsepower motor can often be more effective than a smaller blower with a high horsepower motor.

Evaporative coolers designed for outdoor use - playing fields, swimming pools, construction sites, gardens - tend to be larger than those meant for indoor use. As a general rule of thumb when it comes to evaporative cooler sizing for outdoor use, you should buy the biggest evaporative cooler that you can afford. Keep in mind additional features like the size of the water tank and whether it requires a hose hook up.

Environmental considerations for evaporative cooler sizing

Keep in mind that the overall effectiveness of an evaporative cooler may depend upon other factors in addition to CFM. In particular, the temperature and humidity play a big part in how effective a swamp cooler can be.

Because swamp coolers rely on the natural process of evaporation to cool the air, they work best in arid climates where the air is hot and dry. In humid conditions, the air is already saturated with water so less evaporation and cooling, takes place. The chart below illustrates the effective cooling power of an evaporative cooler relative to humidity and temperature.


Notice how the higher the humidity, the less cooling power an evaporative cooler can deliver. If you are experiencing 80% humidity, not even the largest evaporative cooler is going to be able to provide noticeable cooling relief!

Additional environmental considerations to keep in mind when sizing evaporative coolers include the actual placement of the unit. The higher the CFM of a swamp cooler, the more space it's going to take up in your room - make sure you have the space to accommodate the unit you choose, without impeding traffic and air flow.

Now that you know that evaporative cooler sizing is easy to calculate, you're ready review our selection and choose the perfect unit to fit your needs and ensure that you enjoy cool, comfortable temperatures all summer long!