Quick answer: The HVAC Duct Calculator is a free single-duct sizer. Enter airflow and a friction rate or velocity, and it returns a round duct diameter plus rectangular equivalents. It works in imperial and metric units, and its results are planning estimates only, not a formal ACCA Manual D design.
Free single-duct sizer: enter airflow and a friction rate or velocity, get a round duct size and rectangular equivalents.
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Planning estimate only. This is not an ACCA Manual D formal design and does not replace a licensed engineer. Sizes are not guaranteed — verify before construction.
Equal-friction method. You pick a friction rate (pressure lost per length of duct) or a velocity, and the tool finds the duct size that carries your airflow at that condition, using the Darcy-Weisbach equation with the Colebrook friction factor for galvanized steel ducts in standard air (70°F, sea level).
Rectangular equivalents. Each rectangular size in the table has the same equivalent diameter as the round result under the Huebscher equation, so it carries the same airflow at the same friction rate. A flatter rectangle needs a larger cross-section than the round duct for the same result.
Assumptions. Standard air density, straight galvanized steel duct, no fittings, dampers, or filters. Fittings and long runs add pressure loss that this single-duct calculation does not include — that is one reason the result is a planning estimate, not a finished design.
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Free tool. No account needed, no payment, no paywall.
Yes. It is a free tool. No account needed, no payment, no paywall.
The size of a single duct. Enter airflow and a friction rate or velocity, and it returns a round duct diameter plus rectangular equivalents at the same friction and airflow.
Yes. Imperial uses CFM, inches water gauge, and feet per minute; metric uses litres per second, pascals per metre, and metres per second.
No. This tool is not an ACCA Manual D formal design and does not replace a licensed engineer. Sizes are not guaranteed - verify before construction.