Whole House Fan Size Calculator: How Many CFM You Need?

Every house needs adequate ventilation. It is the job of a whole house fan to provide this ventilation. In milder climates, you can even use whole house fans for cooling without the need for an air conditioning system. In order to do its job right, you obviously have to get the whole house fan size right. We are going to look at what size whole fan do you need.

Namely, you have to figure out how many CFM whole house fan you need. CFM stands for Cubic Feet per Minute; it is a standard metric for the amount of airflow the whole house fan generates. When we talk about the size of whole house fans, we are actually talking about how many CFM can a whole house fan generate.

Example: A small 500 square feet house requires anywhere from 200 CFM to 400 CFM whole house fan size. A big 3000 sq ft house requires much bigger whole house fans, with airflow generating ranging from 1200 CFM to 2400 CFM.

Below, you will find two very useful resources that will help you determine the size of the whole house fan you need. These are:

1. Whole House Fan CFM Calculator. Here you just have to input what size you have (square footage) and the average ceiling height. The calculator will automatically determine the average size of a whole house fan you need (CFM calculated with 4.5 ACH in mind).
2. Whole House Fan Size Chart (100 To 4000 Sq Ft Houses). This is a complete whole house fan sizing chart for all house sizes, ranging from 100 sq ft cabin to 4000 sq ft mansion. We include calculations with 3 ACH, 4.5 ACH, and 6 ACH in mind.

Now, for guidance on how much ventilation your house actually needs we can turn to the US Department of Energy recommendations:

“Whole house fans should provide houses with 3 to 6 air changes per hour.” (DOE on whole house fans)

Based on this 3 ACH to 6 ACH guidance, we can calculate exactly how big of a whole house fan you need.

To understand how to use the calculator and the whole house fan ACH chart below, it is very useful to understand how the size of the whole house fan is calculated based on square footage and ceiling height. Here is what this calculation looks like (it might be a bit complex, the calculator is much simpler to use):

How To Calculate The Whole House Fan Size (CFMs)

ACH stands for Air Changes per Hour. As per DOE recommendation, the whole house fan should be big enough to change all the air in your house 3 to 6 times per hour (every 10 to 20 minutes). In the calculator below, we include an average of 4.5 ACH in order to get an astute estimate of the whole house fan size for most homes.

That simply means that we have to first calculate how much air we have in the house and multiply it by ACH (3 to 6). Our whole house fan should produce the calculated volume of air in order to provide for adequate ventilation and cooling.

Example: How many CFM whole house fan do I need for 1000 square feet? First, we calculate the amount of air in the house. If the house has a standard 8 feet ceiling height, it will contain 8 ft × 1000 sq ft = 8000 cubic feet of air. Now we have to account for air changes per hour:

• Minimum Ventilation (3 ACH): Here we have to multiply 8000 cubic feet by 3; resulting in 24,000 cubic feet per hour requirement.
• Minimum Ventilation (4.5 ACH): Here we have to multiply 8000 cubic feet by 4.5; resulting in 36,000 cubic feet per hour requirement.
• Minimum Ventilation (6 ACH): Here we have to multiply 8000 cubic feet by 6; resulting in 48,000 cubic feet per hour requirement.

Now we know what size whole house fan we need for a 1000 sq ft house; anywhere from 24,000 CFH to 48,000 CFH. The key point here is that the airflow of whole house fans is not expressed in CFH (Cubic Feet per Hour) but in CFM (Cubic Feet per Minute).

To convert CFH to CFM, we need to divide CFM by 60 (since 1 hour has 60 minutes). Here is how to convert this:

Whole House Fan Size (1000 sq ft, 3 ACH) = 24,000 CFH requirement / 60 = 400 CFM.

As we see, we would need a minimum of 400 CFM whole house fan for a 1000 sq ft house. Based on that, you can select the appropriate size of ducts you need. You can check the ductwork size charts here.

Here is the step-by-step process of how to adequately determine the whole house fan size:

1. Determine the square footage. You might have a 500 sq ft, 1000 sq ft, 2000 sq ft house, and so on.
2. Determine the average ceiling height. The standard ceiling height is 8 feet.
3. Calculate the volume of air in your house. You can do this by multiplying the determined square footage by the average ceiling height.
4. Multiply the volume of air in your house by ACH. You can multiply by 3 to get the minimum 3 ACH ventilation, by 4.5 to get the average 4.5 ACH ventilation, or by 6 to get the maximum 6 ACH ventilation.
5. Convert the calculated cubic feet volume from Step 4 to CFM by dividing the result by 60.
6. You get the required size of a whole house fan in CFMs.

Now, this can be quite a lengthy calculation if you do it by hand. To help you out, we have designed a simple-to-use whole house CFM calculator that makes all these calculations automatically:

Whole House Fan CFM Calculator

Just insert the square footage and ceiling height in the calculator, and the calculator will estimate the size of the whole house fan you need (based on 4.5 ACH):

Here is an example of how easy it is to use this calculator:

Let’s say you have a 2000 square feet house and would like to know what size whole house fan you need. Just slide the 1st slider in the calculator to ‘2000’ and the 2nd one to ‘8’ if you have an 8 ft ceiling height.

Result: A 2000 square feet house requires a 1200 CFM whole house fan. If you want better ventilation and cooling, you can use a 1600 CFM fan (6 ACH). If you require less ventilation, you can opt for an 800 CFM fan (3 ACH).

If you don’t want to trouble yourself with all these calculations, you can just check the following whole house sizing chart. It covers all house sizes from 100 sq ft to 4000 sq ft and presumes you have an 8 ft ceiling height:

Whole House Fan Size Chart (100 To 4000 Sq Ft)

With these two resources, you can now adequately determine the size of the whole house fan you need, based on the DOE ACH recommendation for these fans.

As always, if you need any help calculating how many CFM whole house fan you need, you can use the comments below, give us some info on square footage and ceiling height, and we will help you out.