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.
Does this mean if you have a 3-ton unit, you only need a single 18-inch round return? If you want to install multiple returns, should all returns be a minimum of 18-inch round or should it be a cumulative of 1200 cfm?
Hi James, we are always targeting cumulative 1,200 CFM. So, you can go with 1x 18-inch round ducts or 2x 14-inch round ducts. Same with rectangular ducts; two 12×12 ducts can allow for 1,320 CFM airflow. Hope this helps.
I have a 30×40 red iron building with 12 ft ceiling. 2-10×10 roll up doors 1-3 ft. entrance door. Wonting to put a 3.5 ton package unit on side of building run up side about 8 ft go inside about 3ft. Tee off 6 ft on each side for supply then come into side of shop at the package unit for return and filter box what size duct would I need for supply and return?
Hi Willkiker, alright, if you settle for a 3.5 ton unit, you would need 1,400 CFM airflow (400 CFM per ton). There are several choices of return ducts that can handle 1,400 CFM airflow; from 12×22, 8×34, 10×26 rectangular ducts or 18 inch round ducts. You can also combine 2 or 3 smaller ducts, if that is easier in your situation. Hope this helps.
I have 5 8 inch round return ducts. Will this be sufficient for 3.5 ton?
Hi Johnny, alright, let’s calculate that: For 3.5 tons, you would need 1,400 CFM airflow. One 8-inch metal round duct can allow 180 CFM airflow. 5×180 CFM = 900 CFM. So, with five 8-inch round ducts, you are still about 500 CFM short. Eight 8-inch round ducts would be sufficient. Hope this helps.
Hmmm you must be building where space and size doesn’t matter we have gone with ra .05 and sa .10 and return generally 2” bigger.1.5-2 ton 14” return round duct 2.5-3 ton 16” return round duct 3.5-4 18” round duct and 5 ton 20” round duct. I totally agree with more return volume is the best. But we seem to always be forgotten about when architects are drawing up plans and we barely have any space to get properly sized duct installed.we’re close to your suggestion however I don’t think we have ever seen a bag of flex bigger than 20” here . We personally prefer more than one return and if possible one in every room except bathroom closet and kitchen .
Hi Kyle, you are completely right. These are theoretical values. In practice, the architects and their plans will think of ventilation as secondary and you will rarely see openings where you can do a 20″ duct installation. Very good insight.
My experience is 32 years in the field and licensed contractor in Fl