Tools / Duct Pressure

Duct Pressure

Enter airflow and a circular diameter or a rectangular width and height. Velocity pressure is (V / 4005)² in. wg. Friction is Darcy–Weisbach on a straight galvanized duct (ε ≈ 0.0003 ft), no fittings. Rectangular ducts use Huebscher equivalent diameter.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms

Airflow (cfm)

Shape

Diameter (in)

Length (ft)Duct type (velocity band only)

Pressure drop

Velocity1273 FPM

Velocity bandHigh

Velocity pressure0.101 in. wg

Friction rate0.190 in. wg/100 ft

Total drop0.190 in. wg

High for a supply main.

P_v = (V / 4005)² in. wg. Friction is Darcy–Weisbach on a straight duct with no fittings, ε ≈ 0.0003 ft (galvanized): ΔP/100 ft = f × (100 / D) × P_v. Rectangular ducts use Huebscher D_e = 1.30 × (a b)^0.625 / (a + b)^0.25.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms