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How many CFM can a 6 inch duct carry?

A 6 in galvanized duct carries about 111 CFM at a friction rate of 0.1 in.wg/100ft, which is 565 fpm. Stretched flex of the same size carries about 78 before it passes the same friction.

2 min read · Updated 2026-10-09

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The honest answer is that a duct has no single CFM rating. How much it carries depends on how much friction you are willing to accept, and on the material.

What do the common sizes carry at 0.1?

Airflow at 0.1 in.wg/100ft, computed with the app's friction model
Round sizeGalv. steelDuct boardFlex ductVelocity, galvanized
4 in37 CFM31 CFM26 CFM424 fpm
5 in68 CFM57 CFM48 CFM499 fpm
6 in111 CFM94 CFM78 CFM565 fpm
7 in167 CFM141 CFM119 CFM625 fpm
8 in240 CFM202 CFM170 CFM688 fpm

How do you find it in the app?

DuctCalc has no field that solves airflow from a diameter. You work backwards with Check a duct:

  1. Open Check a duct and tap Round duct.
  2. Type 6 in Diameter and choose the Material.
  3. Put a guess in Airflow, say 100, and look at Friction rate: 0.083.
  4. Raise it until the friction reads the rate you design to. 111 CFM gives 0.101 in.wg/100ft.
  5. Read Air velocity and the badge for the same answer.

Friction is not the only limit

The velocity matters too. For a Residential · supply-air branch the recommended band is 500 to 700 fpm with a 900 fpm ceiling. In a 6 in round that band is roughly 98 to 137 CFM. Push 150 CFM through and the badge turns amber at 764 fpm; at 200 CFM it reads Over maximum · resize at 1,019 fpm.

So for a 6 in supply branch, plan on about 100 CFM. That is a common sense figure, not a rating on the duct.

Mistakes to avoid

  • Reading the 111 as a limit. At 0.2 in.wg/100ft the same duct carries more, and it gets louder.
  • Using the galvanized number for flex. Flex drops by about 30 percent in the table above.
  • Ignoring the run length. The friction rate is per 100 ft, so a 15 ft stub and a 60 ft run are not the same job.

Questions and answers

How many CFM will a 6 inch flex duct carry?
About 78 CFM at 0.1 in.wg/100ft when fully stretched. At 100 CFM the friction is 0.162, nearly double the galvanized figure.
How many CFM can a 5 inch duct handle?
About 68 CFM in galvanized at 0.1 in.wg/100ft, about 500 fpm. Flex is about 48.
How many CFM in a 4 inch duct?
About 37 CFM in galvanized at 0.1, which is 424 fpm.
Is there a CFM from diameter button?
No. Use Check a duct and adjust the airflow until the friction rate matches your target.

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