Duct Sizing Calculator — Round Duct Diameter from CFM
Pick the wrong duct size and you either choke the airflow or hear it from every register. This calculator converts your required CFM into the round duct diameter you need, using the velocity your system should run at.
It's the quick field check: CFM ÷ velocity gives the duct cross-section, and that becomes the diameter. Branch runs run quieter than trunks — the calculator lets you adjust for both.
Calculated size
9.6"
Use round duct
10"
Velocity
800 FPM
Trunk ducts run 700–900 FPM; branch runs 400–600 FPM to stay quiet. Sizing up one size beats undersizing every time.
The formula
Duct area (sq ft) = CFM ÷ velocity (FPM). Diameter = √(4 × area ÷ π) × 12. Trunk ducts: 700–900 FPM. Branch runs: 400–600 FPM.
Pro tips
- →Use a lower velocity (500 FPM) for runs near bedrooms — the airflow noise complaints start around 800+ FPM.
- →Round to the next standard size up (4, 5, 6, 7, 8…20). Undersized is the expensive mistake.
- →Pair with the CFM calculator: get the room's CFM first, then size the duct to carry it.
Frequently Asked Questions
What size duct for 400 CFM?+
At 800 FPM, 400 CFM needs about a 9.6" round duct — so a 10" duct. At 500 FPM (quiet branch run), it needs roughly 12".
What's the difference between round and rectangular duct?+
Round duct moves the most air for the least material and resistance. Rectangular fits in tight spaces but needs more cross-section for the same CFM.
How many CFM can a 6-inch duct carry?+
At 800 FPM, a 6" round duct carries about 157 CFM. At 500 FPM, about 98 CFM.
Now imagine the quoting writing itself.
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