01 Set up your duct
02 Result
SUGGESTED ROUND DUCT
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What does this mean?
Velocity is how fast the air moves. Higher speeds can create more noise.
Friction loss is pressure lost as air travels along the duct. Lower values mean less resistance.
Straight-run loss covers only the length you enter. Elbows, fittings, filters and equipment add more resistance.
Calculation method & assumptions
For unlined galvanized metal duct carrying standard air at 20°C and sea level. Air density: 1.204 kg/m³; dynamic viscosity: 1.81 × 10⁻⁵ Pa·s; roughness: 0.09 mm. Round friction uses Darcy–Weisbach and iterative Colebrook; rectangular friction uses the equal-friction round diameter Dₑ = 1.30(ab)⁰·⁶²⁵ / (a+b)⁰·²⁵. Actual rectangular area is used for velocity.
Imperial round sizing uses whole-inch diameters below 8 inches and even diameters from 8 inches upward. Imperial rectangular sizing uses even 2-inch dimensions. Metric round and rectangular sizing use 25-mm and 50-mm increments respectively, with a 6-metre calculation limit. These increments do not guarantee supplier availability. Laminar flow uses 64/Re; transitional flow is interpolated and flagged. Flexible duct, lining, altitude corrections and fitting losses are not included.
Reference: ASHRAE Handbook — Duct Design, friction losses and rectangular ducts. This is a sizing aid; check the complete system against your project requirements.