How Much CFM Can a 4, 6 or 8 Inch Duct Carry?

Updated November 2, 2026 · Foster Tools · How we calculate

A round duct carries its area × air speed. At 700 FPM, a common design speed, a 4-inch duct carries about 61 CFM, a 6-inch about 137 CFM, an 8-inch about 244 CFM and a 10-inch about 382 CFM. For a quieter run at 500 FPM, use about 70% of those numbers.

The formula

CFM = π ÷ 4 × (diameter in inches ÷ 12)² × velocity (FPM)

A 6-inch duct has an area of π ÷ 4 × 0.5² = 0.196 ft². At 700 FPM that's 0.196 × 700 = 137 CFM. Double the diameter and the area, and the airflow, goes up four times: a 12-inch duct carries 550 CFM at the same speed.

CFM by duct size

Friction is in inches of water column per 100 feet of straight duct. Small ducts lose much more pressure at the same speed, which is why a 4-inch run at 700 FPM loses about three times what a 10-inch run does.

Which column to use

• 500 FPM: bedroom supplies and returns where noise matters.
• 700 FPM: a common middle ground for supply branches.
• 900 FPM: the upper end, usually for trunks rather than runs that end at a register near someone's bed.

Your system's design (a Manual D layout, if you have one) sets the real targets. These columns are rules of thumb for checking whether a duct you already have is in the right range.

Checking an existing duct

1. Measure the duct's inside diameter. Flex duct is sold by its inside diameter.
2. Find the row in the chart. A 6-inch branch supplying 180 CFM is running at about 900 FPM, at the top of the range.
3. Look at the run. A long run with several elbows, or flex that's compressed or sagging, carries less than the chart.
4. Decide: upsize the run, add a second run, or accept the airflow it gives.

Common mistakes

• Using the duct's outside size. Insulated flex is much bigger on the outside. The airflow depends on the inside diameter.
• Reading a chart built for a different velocity. Two charts can disagree and both be right; check which speed or friction rate each one uses.
• Ignoring fittings. Each elbow adds the friction of several feet of straight duct.

These are equal-velocity figures for standard air in smooth galvanized duct. They are a starting point, not a Manual D design; flex duct and long or fitting-heavy runs carry less.