Enter the airflow in CFM and a friction rate or target velocity. You'll get the recommended round duct diameter and the equal-friction rectangular equivalents — then design and price that exact fitting.
Every size here is fabricated to order — not rounded to a stock-shelf sizePresets set a typical friction rate & velocity — adjust either below.
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| Round size | Velocity | Friction /100 ft |
|---|
| Rectangular (W×H) | Equiv. dia. | Velocity |
|---|
Round diameter uses the ASHRAE equal-friction equation for standard galvanized round duct and standard air:
D = (0.109136 × CFM1.9 ÷ friction)1/5.02. The velocity method uses
V = 183.35 × CFM ÷ D². Rectangular equivalents come from the ASHRAE equivalent-diameter relationship
De = 1.30 × (W·H)0.625 ÷ (W+H)0.25, solved for each standard duct height. Results are rounded up to the next standard duct size. These are planning figures — confirm the full system against a Manual D / SMACNA calculation for your job.
Duct size is driven by two things: the airflow the run must carry (in CFM) and how much friction loss or air velocity you're willing to accept. The equal-friction method sizes every run in the system to the same pressure drop per 100 feet of duct — typically 0.08–0.10 in. wg for residential and light-commercial supply, and a little lower (0.06–0.08) for return air so the return runs larger and quieter. Enter your CFM above and the calculator returns the round diameter, then rounds up to the next duct size that's actually made.
A quick reference for common round sizes at a standard 0.10 in. wg friction rate: a 6 inch round duct carries roughly 110 CFM, 8 inch about 240 CFM, 10 inch about 430 CFM, 12 inch about 680 CFM, and 14 inch around 1,000 CFM. Change the friction rate or switch to the velocity method above to size for your own target.
When headroom forces a rectangular duct, you want the rectangle that moves the same air at the same friction as the round size — its equivalent diameter. The calculator lists those rectangular equivalents for each standard duct height, so a 10 inch round becomes 8×8, 10×6, 12×5, and so on. Pick the one that fits your cavity.
Every size this calculator returns can be fabricated to order in galvanized steel, aluminum or stainless — you're never forced to the nearest stock-shelf size. Hit Design & price on any result to open the duct designer with that size loaded, see the live price and 3D drawing, and order online. For background reading, see how to size ductwork, CFM duct sizing, duct velocity, and return-air duct sizing.