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Evaporative Cooler Duct Sizing [Explained]

Living in a low humidity area in the summertime is pretty challenging. It’s even more so if your evaporative cooler isn’t giving the best performance. Maybe this is the right time to attach ductwork to your cooler.

But how to measure evaporative cooler duct sizing?

First, calculate the cooler’s CFM rate. Get your ductwork according to the CFM measurement of the cooler. For 1 ton of air, you will need 400 CFM space. You can use a rectangular or circular duct. But make sure it has enough space to contain all the air.

That was just the preview. We have explained the measurement process in detail in this article.

Let’s quickly jump right into the details!

Calculating CFM Of The Cooler

To attach ductwork to your cooler, you need to know the CFM rate first. Evaporative coolers are calculated by cubic feet of air per minute (CFM). This determines how many units of air it can pump in 1 CFM.

The CFM number of the cooler comes with the user manual. If you can’t find it, calculate the CFM manually.

Multiply the square foot of your housing with the average height. Then divide it by 2. 

Suppose, you live in a 1700 sq. ft. house. The average height from floor to ceiling is 8 feet. Which is (1700 sq.ft. x 8 ft)/2= 6800 CFM. It means you require an evaporative cooler that is of at least 6800 CFM. 

Attaching ductwork can restrict some airflow in the area. But if you have a good home ventilation system, it won’t be a problem.

Proper Measurement for Duct Sizing

Now that you know your cooler’s CFM rate, let’s move on to calculating duct size.

For 1 ton of air, you will need 400 CFM worth of space. It is to fit all that air in the duct. It can’t be oversized or smaller. 

Using large flexible and insulated ductwork is common in the installation process. It depends on what CFM cooler you are using. This table shows the ductwork size based on the cooler CFM:

Air volumeRectangular duct height (inches)Equivalent circular duct

These are the available sizes for the ductwork according to the CFM rate. For some ductwork, you may not find the exact size you are looking for. 

For example, 500 CFM needs ductwork that is longer than 4 inches. Similarly, it doesn’t need ductwork that is 12 inches long. This is why some boxes in the chart don’t have measurements.

Most evaporative coolers for bigger residences use 18×18 or 20 x 20 ductwork. Because it’s the common measurement for ductwork through floor trusses or ceilings.

For smaller areas, you don’t have to go through much hassle. You can simply measure the distance of the cooler from the room. Later, adjust the ductwork to the required size.

Evaporative Cooler Duct Installation

Hopefully, you got the right measurement for your ductwork. To install it with your swamp cooler you are gonna need some duct kits. Here’s what you need-

  • working gloves
  • Screw gun
  • Duct tapes
  • Jigsaw
  • Self-tapping sheet-metal screws (10-12)

If you have gathered the tools, let’s get started.

Step-1: Attach Flange To The Cooler

You have to first attach the metal flange to the cooler. Take 5-6 self-tapping sheet metal screws and hold the flange over the cooler. Attach the screws to the edges of the flange with a screw gun carefully.

Resize the height of ductwork if necessary. You can cut the ductwork with a Dremel to the measured height.

Step-2: Attach Ductwork To The Flange

The next step is connecting the duct to the metal flange. You will see that the ductworks come in two layers. 

You have to attach the duct collar to the inner part of the insulation. Both ends of the ducts have to be connected to the duct collar.

Once you attach them, use duct tape to secure the connection. It’s time to attach the flared end of the ductwork to the metal flange.

Connect the collar to the flange. Use duct tape to wrap the connection 5-6 times. When done, just make sure it’s properly attached.

Step-3: Attaching The valve

The other end of the duct is going to be attached to a valve. The valve gets attached to the wall. It supplies the air from the cooler to the room.

This is why now attach the valve with the duct collar. Use 4-5 self-tapping sheet-metal screws to connect the collar with the valve. 

Wrap the joint with duct tape 5 to 6 times afterward. This will ensure that no air comes out of the joining point.

Step-4: Make The Ventilation Space

To attach the ductwork to a room, you need to have a ventilation space. The space should be the size of the valve’s diameter.

Cut a hole to the wall in the size of the valve’s diameter. Use a jigsaw to cut the wall. You can use the jigsaw to cut the ductwork as well. If you don’t have a jigsaw, you can use a drill machine.

Step-5: Attaching Valve To The Wall 

It’s time to position the valve to the ventilation space you just made. Stick it to the other end of the wall. Use screw guns to attach the valve to the wall.

That’s it! You are done with installing ductwork to your evaporative cooler.


Question: Are evaporative coolers only suitable for dry climates?

Answer: No, you can use it in outdoor spaces like garages, backyard, even in humid conditions. But using them indoors in humid places is not a good idea. It makes the air muggy. It might cause swelling to your furniture too.

Question: How long should I run the pump in the evaporative cooler?

Answer: As long as there’s water in the pan, running the pump wouldn’t be a problem. If the water dries out and the pump keeps running, it will overheat the pump. This causes the pump to burn out early.

Question: Does an evaporative cooler work as well as an air conditioner?

Answer: No. An evaporative cooler decreases the temperature in the air by a significant amount. But it can’t maintain a certain temperature like air conditioning does. They can only replace it in certain climates.


That’s everything we had regarding evaporative cooler duct sizing. Hopefully, you have a better idea about the measurements now.

Reach out to get professional help anytime you need it. 

Finally, good luck with fixing your problems!