Enter the airflow and pick where the duct runs, and this tool sizes it both ways, a standard round diameter and a rectangular equivalent, then tells you the velocity you actually get. It uses the velocity-reduction method for a fast check on the truck or at the takeoff. For a full design, follow up with equal-friction and Manual D. Runs in your browser.
Duct area equals airflow divided by velocity. Put CFM over feet-per-minute and you get square feet of free area; multiply by 144 for square inches. From there a round diameter or a rectangular width and height that meets that area will carry the air at your target speed. That is the whole calculation this tool runs, and it is enough to confirm a takeoff size or sanity-check a plan on site. Full system design layers in total external static pressure, fitting losses, and the blower's curve, which is what the equal-friction method and Manual D handle.
Size a duct too small and you are pushing air fast: more noise at the registers, more static pressure fighting the blower, less airflow delivered. Size it generously and the system runs quiet and efficient but costs more material and space. The application presets here reflect the ranges the trade actually uses: faster in trunks where a little noise is tolerable, slower in returns and near living space where it is not. Drop the custom velocity and watch the duct grow, that trade is the design in miniature.
For the same capacity, round duct has less surface area rubbing on the airflow, so it moves air with less friction and less sheet metal. It is the efficient default when the framing has room. Rectangular wins where it has to: tight joist bays, low soffits, above a dropped ceiling. This calculator gives both every time so the answer is "here is the round size, and here is the rectangle that fits your 8 inch space," not one or the other.
Duct work is measured, cut, and billed by the job. When the install is done, FieldForge lets you say what went in, the runs, the registers, the labor, and the invoice builds from your price book with a card payment link texted to the customer. Need a clean document today instead? The free HVAC invoice generator prints one in the browser.
About a 9 inch round or 8 by 8 rectangular at 900 fpm supply-trunk velocity. Slower target, bigger duct.
Match the free area. This tool shows the rectangular width for whatever height fits your framing.
Roughly 700 to 900 fpm supply trunks, 600 to 700 branches, 500 to 700 returns in homes.