Duct Size Calculator
Equal-friction sizing for galvanized sheet metal duct. Enter airflow and friction rate; pick a height for the rectangular size.
How it works
Friction per 100 ft = 0.109136 × CFM^1.9 ÷ D^5.02
Rectangular equivalent (Huebscher) = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25
The calculator solves for the round diameter that carries your airflow at your friction rate, rounds up to the next standard size, then finds the narrowest rectangle (in 2-inch steps) at your chosen height with the same or better friction. Results assume standard air and smooth galvanized duct.
Notes for the field
- Flex duct has much higher friction than sheet metal, even fully stretched. Size flex up, and pull it tight.
- The friction rate should come from the blower's available static pressure divided by the total effective length (Manual D). 0.06–0.10 in. w.c. per 100 ft is a common residential range.
- Keep residential supply branches under roughly 700–900 FPM to avoid noise.
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Results are estimates for field reference. Always confirm against the equipment manufacturer's charging chart, installation instructions and local code. Found a problem with this tool? Tell us.