DUCT
SIZING
Equal friction or constant velocity. Round, rectangular, or flat oval. Real Darcy-Weisbach + Colebrook physics — the same math behind the ASHRAE friction chart — with every step and every citation shown, so you can defend the number, not just read it.
Runs in your browser · Your inputs never leave it · Also free: pipe sizing · conduit fill
The friction chart, computed — not eyeballed
The classic duct friction chart is a graph of the Darcy-Weisbach equation with the Colebrook friction factor, drawn for standard air in average galvanized duct. This calculator computes that math directly: your airflow, your duct material's roughness, your altitude and temperature — no chart-reading error, no "close enough" from a duct wheel.
Methods
Equal friction sizes the duct so friction loss per 100 ft matches your design rate — the standard method for low-pressure supply and return trunks. Constant velocity sizes to a target velocity — what you want for flues, dust collection, or noise-critical runs. Check existing size works backwards from a duct you already have. Rectangular and flat-oval sections use the ASHRAE equivalent-diameter equations (Huebscher), and the calculator flags high aspect ratios that waste metal and pressure.
What's code and what's guidance
Duct friction physics is engineering, not statute. Velocity limits and friction-rate targets are industry design guidance (ASHRAE / SMACNA practice) — the calculator cites them as guidance and never dresses a rule of thumb up as a code section. When a result exceeds a guidance limit, you see it flagged with its source.
Sizing ducts for a Revit model?
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