How do I calculate duct size? Residential supply trunks typically sized for 700 ft/min; branches ~600. Enter your numbers above and the result updates instantly. For example: Airflow required: 400 CFM · Duct type: Round · Target velocity: 700 ft/min → Round duct diameter: 11 in.
Is this Duct Size Calculator free? Yes — completely free with no sign-up or paywall. It runs entirely in your browser and keeps working offline; the numbers you enter never leave your device.
How accurate is it? It uses the standard published formula for duct size. Results are estimates for planning — not financial, tax, medical, engineering, or legal advice.