Flex & Round Duct Sizing Chart (4, 6, 8, 10, 12 Inch Duct Max CFM)

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.

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 SizeMaximum CFM
(0.05 Friction Rate)
Maximum CFM
(0.1 Friction Rate)
4 inch20 CFM20 CFM
5 inch50 CFM50 CFM
6 inch75 CFM80 CFM
7 inch110 CFM120 CFM
8 inch160 CFM170 CFM
9 inch225 CFM230 CFM
10 inch300 CFM300 CFM
12 inch480 CFM500 CFM
14 inch700 CFM740 CFM
16 inch1000 CFM1050 CFM
18 inch1300 CFM1400 CFM
20 inch1700 CFM1875 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 SizeMaximum CFM
(0.06 Friction Loss)
Maximum CFM
(0.1 Friction Loss)
4 inch20 CFM20 CFM
5 inch50 CFM70 CFM
6 inch85 CFM115 CFM
7 inch125 CFM170 CFM
8 inch180 CFM240 CFM
9 inch240 CFM320 CFM
10 inch325 CFM420 CFM
12 inch525 CFM680 CFM
14 inch750 CFM1050 CFM
16 inch1200 CFM1450 CFM
18 inch1500 CFM2000 CFM
20 inch2000 CFM2600 CFM

Maximum CFM for 6 inch duct?

85 CFM at 0.06 friction loss or 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

CFMDuct Size
(0.06 Friction Loss)
Duct Size
(0.1 Friction Loss)
300 CFM10 inch9 inch
400 CFM11 inch10 inch
600 CFM14 inch12 inch
800 CFM16 inch14 inch
900 CFM16 inch14 inch
1000 CFM16 inch14 inch
1200 CFM16 inch16 inch
2000 CFM20 inch18 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.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

Leave a Comment

DMCA.com Protection Status