CFM to Duct Size Calculator (Airflow to Duct Diameter)

Each duct run is sized independently from its own entered CFM and duct category (Supply Trunk, Supply Branch, Return Trunk, or Return Branch) — this calculator does not attempt to auto-distribute a whole-system CFM across runs, since that's a genuine Manual D design step. Each category targets its own recommended (quieter, more conservative) velocity — Supply Trunk 700 FPM, Supply Branch 600 FPM, Return Trunk 600 FPM, Return Branch 400 FPM — planning limits informed by common ACCA Manual D velocity guidance, rather than the higher maximum each category still commonly allows. Required duct area is CFM ÷ target velocity, converted to a round diameter, then rounded UP to the nearest standard stock size (4 through 24 in). The velocity shown for the sized duct is recalculated at that actual standard size, which is usually a little under the target since sizes snap up in whole-inch steps. A run whose raw diameter would exceed 24 in is flagged rather than capped, since that's outside this calculator's modeled single-round-duct range and likely needs splitting into parallel runs or a professional design. "Suggested Total System CFM" (tonnage × CFM/ton) is an optional, informational cross-check only — it is never substituted into any individual run's own CFM. This is a simplified velocity-only sizing shortcut, not a full ACCA Manual D equal-friction design — a real Manual D calculation sizes duct from design airflow, friction rate, duct material/roughness, and total system static pressure, not velocity limits alone, and also accounts for duct length, fitting/elbow friction loss, and total effective length that this shortcut does not model. Get a full duct design from a licensed HVAC contractor before fabrication or installation.