Duct Sizing Calculator
Enter the airflow (CFM) and duct location — get the recommended round duct diameter at the correct design velocity.
| Required duct area |
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Common round-to-rectangular equivalents
Many residential installs use rectangular duct in tight joist bays where round doesn't fit. These are commonly published approximate equivalents (same cross-sectional area, not accounting for the added corner friction rectangular duct has) for a few frequent sizes this calculator tends to return:
| Round diameter | Common rectangular equivalent |
|---|---|
| 6" | 4" × 10" |
| 8" | 4" × 16" or 6" × 10" |
| 10" | 6" × 14" or 8" × 10" |
| 12" | 8" × 14" or 10" × 12" |
How to use this calculator
Enter the CFM (cubic feet per minute) of airflow the duct needs to carry, and pick its location — main trunk, supply branch, or return. Each location has a different standard target velocity, since a trunk line carrying air the full distance needs a higher velocity target than a short branch run, and returns typically run slightly slower than supply to control noise.
Why velocity, not just CFM, determines duct size
The same CFM can be moved through a smaller duct at high velocity or a larger duct at low velocity — the
trade-off is noise and static pressure. Too high a velocity for the duct's role causes noticeable air
noise; too low wastes material and space unnecessarily. This calculator solves for the diameter that hits
the standard target velocity for the duct's specific role, using the formula
diameter = √(4 × area / π), where area comes directly from CFM ÷ velocity.
Worked example
600 CFM through a main trunk (900 fpm target): required area is 600/900 = 0.667 sq ft = 96 sq in, which works out to an 11.1-inch diameter round duct — round up to the nearest manufactured size (12-inch) in practice.
Frequently asked questions
What if I'm using rectangular duct, not round?
Convert the required area to a rectangular equivalent (length × width = the same square-inch area) rather than using the round diameter directly — rectangular duct sizing charts account for the added friction of corners, so a straight area match is an approximation, not an exact equivalent.
Should I always round up to the next duct size?
Yes — rounding down increases velocity above the target, which raises both noise and static pressure. Manufactured duct comes in fixed size steps, so round up to the nearest available size above the calculated diameter.
Why does a return duct use a lower target velocity than supply?
Return air noise is more noticeable to occupants than supply noise in most home layouts, so return ducts are conventionally sized for lower velocity (more airflow per size step) to keep them quieter.
Based on standard residential design velocity targets. Reference only — a full duct design also accounts for total system static pressure and friction rate over the full run length. Consult a licensed HVAC contractor for final specifications.