Duct Sizing Guides

The duct sizing equation A = Q / V broken down step by step, with worked supply trunk and branch examples and a CFM to duct size chart.

Huebscher equivalent diameter, what aspect ratio really costs in metal and pressure drop, and a round to rectangular lookup table.

Why flex runouts get undersized, the friction rate to design them at, and how much compression adds to the pressure drop.

Max FPM chart for every application: 600–900 FPM residential supply, 1,200–1,800 FPM commercial trunks, 2,500+ FPM industrial, with matching friction rates.

Stock round duct sizes from 4 to 60 inches, sheet metal gauge by size, rectangular stock dimensions, and how to round 13.5 in to the right 14 in order.

The Darcy-Weisbach method end to end — velocity, velocity pressure, friction factor, friction rate and run total — with a worked 4,000 CFM commercial trunk.

What in. w.g. per 100 ft actually represents, the common design ranges by system type, and how to derive a defensible rate from available fan static pressure.

The C × Pv method, a reference coefficient table for elbows, tees, take-offs and dampers, and how to convert a coefficient to equivalent length.

The flow physics behind fitting losses, each fitting family priced in feet of straight duct, and before/after swaps that recover static pressure for free.

The default commercial sizing method step by step, a full reducing-trunk schedule at 0.10 in. w.g./100 ft, and the four cases where equal friction is the wrong choice.

Total effective length per route, a worked four-path comparison showing why the longest run often is not critical, and the field signs you picked the wrong one.

What counts as external, a line-by-line TESP worksheet for a rooftop VAV system, working backwards to available duct static, and the mistakes that oversize fans.

Wall roughness and design friction-rate ranges for galvanized metal, flexible duct, fiberglass duct board and duct liner, with guidance on where each material belongs.

Five reusable worked examples — 10,000 CFM office trunk, VAV branch, kitchen exhaust, elbow before/after, and a round to rectangular conversion — each with interpretation.