1. Start with required airflow
Each duct run needs an assigned CFM from the room/load and air-distribution design. A duct diameter does not establish airflow, and a whole-system CFM should not be copied to every branch. Identify supply, return, and exhaust paths separately.
2. Convert airflow to preliminary area
Velocity (FPM) = CFM ÷ duct area (ft²)
The chosen velocity is a project criterion, not a universal number. It must balance geometry, sound, pressure loss, and available fan pressure.
3. Select an available size and recalculate
For a 400 CFM branch with a preliminary 800 FPM criterion, area is 400 ÷ 800 = 0.50 ft². A selected round or rectangular duct will have its own actual area, so recalculate actual velocity instead of assuming the preliminary value is retained.
4. Check the whole path
| Check | Why it matters |
|---|---|
| Friction in straight duct | Consumes part of the fan-pressure budget. |
| Fittings and transitions | Elbows, takeoffs, dampers, and transitions add losses beyond straight length. |
| Terminals and filters | Grilles, diffusers, coils, and filters add external static pressure. |
| Actual fan performance | The fan must deliver the required airflow at the total system resistance. |
| Balancing and sound | A technically possible size can still create noise or poor room distribution. |
Use a documented design method
ASHRAE describes equal-friction and other duct design approaches. The appropriate method depends on the system, route, equipment, and project constraints. A calculator can provide a transparent geometry or straight-run check, but it does not replace a complete duct design or field verification.
FAQ
Can one velocity fit every duct run?
No. Main ducts, branches, terminals, noise-sensitive spaces, available geometry, and pressure budget can require different criteria.
Is a round duct always the best choice?
No. Round duct is often straightforward to check, but rectangular geometry may be necessary where the route is shallow or constrained. Compare actual area, equivalent behavior, fittings, and construction.