The flex duct sizing chart and round duct sizing chart below are used by HVAC technicians, handyman services and homeowners to quickly select a supply duct size and return air duct size for each section of your ductwork design. It’s that easy.
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How Does it Work?
We’ve used the formula for duct sizing and compiled quick, at-a-glance tables for you to use as you design your own or a client’s ductwork.
What’s the formula? As a refresher, it is assumed that you have used Manual J and/or related software to determine:
- Total load for the home or building in BTUs.
- Load for each room.
- Each room’s percentage of the total – the formula to determine this is to divide the room load by total load. For example, if the total load is 34,000 BTU and the room load is 4,100, its percentage is 0.121 or 12%.
- To determine needed CFM per room or zone, multiply the % for each room by the total CFM for the HVAC system. In the example above, if total CFM is 1500 and the room needs 12% of that, the room needs a duct that can carry 180 CFM.
OK, we covered that simply to make sure that we’re all on the same page. Now, let’s examine the ductwork CFM chart for round flex and round metal duct. They are slightly different due to a small variance in friction rates. This leads to slightly different results in the duct sizing tables.
Round Flex Duct CFM Chart
Round flex duct is easy to work with and typically comes insulated for convenience. The downside is that it can be more easily damaged and doesn’t last as long. And if it is compressed, not fully stretched, airflow will be negatively and significantly affected.
This flex duct size and CFM chart covers all common sizes at two friction rates. The numbers are those you would obtain using a duct calculator, aka a ductulator.
Pro tip: Remember to use a duct calculator for the kind of duct you are using. You might get incorrect results using a calculator designed for metal if you’re installing flex, for example.
FAQs below the table answer common sizing questions.
Round Flex Duct CFM Chart:
| Round Flex Duct Size | Maximum CFM (0.05 Friction Rate) | Maximum CFM (0.1 Friction Rate) |
| 4 inch | 20 CFM | 20 CFM |
| 5 inch | 50 CFM | 50 CFM |
| 6 inch | 75 CFM | 80 CFM |
| 7 inch | 110 CFM | 120 CFM |
| 8 inch | 160 CFM | 170 CFM |
| 9 inch | 225 CFM | 230 CFM |
| 10 inch | 300 CFM | 300 CFM |
| 12 inch | 480 CFM | 500 CFM |
| 14 inch | 700 CFM | 740 CFM |
| 16 inch | 1000 CFM | 1050 CFM |
| 18 inch | 1300 CFM | 1400 CFM |
| 20 inch | 1700 CFM | 1875 CFM |
What is the maximum CFM for 4 inch duct?
For a 4 inch duct, maximum CFM is 20.
What is the maximum CFM for 6 inch duct?
Typcially, the max cfm for 6 inch duct is 80 cfm.
12 inch flex duct CFM?
Using the chart, you’ll see that for a 12” round flex duct, maximum CFM is 480 CFM or 500 CFM depending on the friction rate.
Duct size for 115 CFM.
Find the CFM closest to 115 in the table above. It will be found in different columns depending on the friction rate. If there isn’t an exact match, go up to the duct size.
The size will be either 6” or 7” depending on the friction rate.
Round Metal Duct CFM Chart
Metal ductwork is tried and true, and it serves the system well when it is wrapped to eliminate air leaks and insulated to protect system efficiency.
Terminology tip: Friction Rate and Friction Loss refer to the same measurement.
Round Metal Duct CFM Chart:
| Round Metal Duct Size | Maximum CFM (0.06 Friction Loss) | Maximum CFM (0.1 Friction Loss) |
| 4 inch | 20 CFM | 20 CFM |
| 5 inch | 50 CFM | 70 CFM |
| 6 inch | 85 CFM | 115 CFM |
| 7 inch | 125 CFM | 170 CFM |
| 8 inch | 180 CFM | 240 CFM |
| 9 inch | 240 CFM | 320 CFM |
| 10 inch | 325 CFM | 420 CFM |
| 12 inch | 525 CFM | 680 CFM |
| 14 inch | 750 CFM | 1050 CFM |
| 16 inch | 1200 CFM | 1450 CFM |
| 18 inch | 1500 CFM | 2000 CFM |
| 20 inch | 2000 CFM | 2600 CFM |
Maximum CFM for 6 inch duct?
85 CFM at 0.06 friction lossor 115 CFM with a 0.1 friction loss. When friction loss is lower, then a lower CFM will do the job.
How many cfm for 8 inch round?
180 CFM with a 0.05 friction rate and 240 CFM at 0.1 friction rate. That’s the 8” flex duct CFM.
How many CFM is a 10 round duct?
325 CFM to 420 CFM. Again, knowing the friction loss is essential to sizing a duct and knowing how many CFM are required to meet the room’s load calculation.
Duct Size for 300 – 2000 CFM
| CFM | Duct Size (0.06 Friction Loss) | Duct Size (0.1 Friction Loss) |
| 300 CFM | 10 inch | 9 inch |
| 400 CFM | 11 inch | 10 inch |
| 600 CFM | 14 inch | 12 inch |
| 800 CFM | 16 inch | 14 inch |
| 900 CFM | 16 inch | 14 inch |
| 1000 CFM | 16 inch | 14 inch |
| 1200 CFM | 16 inch | 16 inch |
| 2000 CFM | 20 inch | 18 inch |
2000 CFM duct size?
18 inch or 20 inch duct based on friction loss. See the column above corresponding to friction loss in your system.
Size Up or Down?
Having extra capacity in your ductwork is OK, as long as the duct isn’t oversized by more than one size.
But not having enough CFM capacity in the ductwork can produce significantly reduced performance, efficiency and durability in the system.
Therefore, when choosing duct size, always round up rather than down, even if you are just a few CFM over the limit. For example, consider the 0.1 Friction Loss column of the Round Metal Duct CFM Chart.
If you need 125 CFM, choose a 7” duct capable of 170 CFM rather than sizing down to a 6” duct with maximum CFM of 115.
What About Trunks?
Trunks usually serve at least two supply or return ducts.
To size supply trunks, add together the CFM totals for each supply branch the trunk will feed. In other words, if you need 95 CFM at one register and 190 at the second, the total is 285 CFM. Find the supply duct size that will handle 285 CFM – and remember to round up if necessary.
For return air trunk size, follow the same formula – add up the total CFM from the returns, and choose the correct return air duct size.
