What size return ducts do I need?
Adequately sizing return ducts is (fairly) easy. We usually need to figure out CFMs based on HVAC tonnage, and select the right round or rectangular duct size. We will show you exactly how to determine the return duct size. Further on, you will also find a very helpful return duct size chart for 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, and 6 ton units (CFMs, return round duct size, return rectangular duct size).
Alright, all return duct sizing starts with this one HVAC rule of thumb:
For every ton of cooling or heating, we need 400 CFM of airflow. This is based on the standard 500 FPM airflow velocity.
Using 1 ton = 400 CFM, we can quickly see what airflow the return ducts have to facilitate. Here are just a few examples:
- For a 2-ton unit, we need a return duct that can facilitate 800 CFM airflow.
- For a 3-ton unit, we need a return duct that can facilitate 1,200 CFM airflow.
- For a 4-ton unit, we need a return duct that can facilitate 1,600 CFM airflow.
Easy, right?
Figuring out the CFMs for the HVAC tonnage is Step 1.
Step 2 is to decide whether you want to use round or rectangular ducts. Step 3 is just to match the CFMs with the right size return ducts that can facilitate this airflow. Alright, let’s put this in a structured form:
How To Calculate Return Duct Size
Here is the step-by-step guide you can use (the all-encompassing chart is below the guide):
- Multiply the tonnage of your HVAC unit by 400 to get the CFMs. Example: 3-ton unit will require 3 × 400 CFM = 1,200 CFM airflow.
- Specify the shape of the ducts you are using: Return ducts can be either round or rectangular.
- Match the calculated CFMs from Step 1 with the size of the ducts. You should use these duct CFM sizing charts. Example: For 1,200 CFM airflow, you can either use 18 inch round ducts or 20×12, 22×10, 30×8 rectangular ducts (because these can facilitate an airflow of 1,200 CFM or above).
Here is an example of how this Step 3 matching goes (screenshot from duct CFM charts for 12 inch rectangular ducts):
That’s pretty much it.
We have done the return duct size calculations for HVAC units ranging from 1 ton to 6 tons. Here they are, summarized in a neat chart:
Return Duct Size Chart (1-6 Ton Units)
| HVAC Tonnage: | Recommended Airflow: | Round Duct Size: | Rectangular Duct Size: |
| 1 Ton Unit: | 400 CFM | 12 Inch Round Duct | 10×10, 12×8, 18×6 |
| 1.5 Ton Unit: | 600 CFM | 14 Inch Round Duct | 12×12, 14×10, 18×8, 24×6 |
| 2 Ton Unit: | 800 CFM | 16 Inch Round Duct | 14×12, 16×10, 22×8, 24×6 |
| 2.5 Ton Unit: | 1,000 CFM | 16 Inch Round Duct | 18×12, 20×10, 26×8 |
| 3 Ton Unit: | 1,200 CFM | 18 Inch Round Duct | 20×12, 22×10, 30×8 |
| 3.5 Ton Unit: | 1,400 CFM | 20 Inch Round Duct | 22×12, 26×10, 34×8 |
| 4 Ton Unit: | 1,600 CFM | 20 Inch Round Duct | 24×12, 30×10 |
| 4.5 Ton Unit: | 1,800 CFM | 22 Inch Round Duct | 28×12, 32×10 |
| 5 Ton Unit: | 2,000 CFM | 22 Inch Round Duct | 30×12, 36×10 |
| 6 Ton Unit: | 2,400 CFM | 24 Inch Round Duct | 34×12 |
As you can see from the chart, you can use these return duct sizes:
- For a 1-ton unit, we use 12 inch round returns ducts or 10×10, 12×8, and 18×6 rectangular returns ducts. These ducts can facilitate at least 400 CFM airflow.
- For a 1.5-ton unit, we use 14 inch round returns ducts or 12×12, 14×10, 18×8, and 24×6 rectangular returns ducts. These ducts can facilitate at least 600 CFM airflow.
- For a 2-ton unit, we use 16 inch round returns ducts or 14×12, 16×10, 22×8, and 24×6 rectangular returns ducts. These ducts can facilitate at least 800 CFM airflow.
- For a 2.5-ton unit, we use 16 inch round returns ducts or 18×12, 20×10, and 26×8 rectangular returns ducts. These ducts can facilitate at least 1,000 CFM airflow.
- For a 3-ton unit, we use 18 inch round returns ducts or 20×12, 22×10, and 30×8 rectangular returns ducts. These ducts can facilitate at least 1,200 CFM airflow.
- For a 3.5-ton unit, we use 20 inch round returns ducts or 22×22, 26×10, and 34×8 rectangular returns ducts. These ducts can facilitate at least 1,400 CFM airflow.
- For a 4-ton unit, we use 20 inch round returns ducts or 24×12 and 30×10 rectangular returns ducts. These ducts can facilitate at least 1,600 CFM airflow.
- For a 4.5-ton unit, we use 22 inch round returns ducts or 28×12 and 32×10 rectangular returns ducts. These ducts can facilitate at least 1,800 CFM airflow.
- For a 5-ton unit, we use 22 inch round returns ducts or 30×12 and 36×10 rectangular returns ducts. These ducts can facilitate at least 2,000 CFM airflow.
- For a 6-ton unit, we use 24 inch round returns ducts or 34×12 rectangular returns ducts. These ducts can facilitate at least 2,400 CFM airflow.
With this return duct size chart, you will have no problems figuring out how big return duct you need. If you need any help, you can give us some info in the comment section below, and we’ll advise you on how to move forward.