About this tool
The Duct Size Calculator is a free, in-browser HVAC tool that sizes ductwork by the velocity method: it finds the cross-sectional area a duct needs to carry a given airflow at a chosen air speed. The required area in square feet is simply CFM ÷ velocity (fpm); multiplied by 144 that becomes square inches. From the area a round duct diameter follows as d = √(4·A/π) = √(1.2732·A), rounded up to the next standard round size (4, 5, 6, 7, 8, 9, 10, 12, 14, 16 in and up).
All calculations run locally in your browser, so nothing you enter is uploaded. For a rectangular duct you fix one side and the tool computes the other from the same required area (b = A ÷ a), then reports the ASHRAE equivalent-round diameter — the round duct with the same friction loss and airflow — using De = 1.30·(a·b)^0.625 ÷ (a+b)^0.25. It also shows the actual velocity in the nearest standard round duct, so you can see how rounding up affects air speed.
Typical target velocities guide the choice: residential supply mains run about 700–900 fpm, branch runs near 600 fpm, and returns 500–700 fpm, with lower speeds for quieter operation. This is a practical velocity-method sizing aid; a full design should also check the total friction rate (often about 0.1 in. w.g. per 100 ft) and account for fittings, but the velocity method is the standard quick way to pick a duct size.