Calculate air velocity and friction pressure drop for rectangular and circular HVAC ducts. Uses the Darcy-Weisbach equation with real material roughness values. SI units.
Enter duct dimensions and airflow to calculate velocity and pressure drop.
Aspect ratio limit: Keep width:height ratio ≤ 5:1 for reliable results. Above this ratio, pressure drop calculations become unreliable.
Enter duct diameter and airflow to calculate velocity and pressure drop.
Pressure drop calculated using: ΔP/L = f × (1/D) × (ρV²/2), where f is the Darcy friction factor, D is hydraulic diameter, ρ is air density (1.2 kg/m³), and V is velocity.
Friction factor solved iteratively using the Colebrook-White equation: 1/√f = −2 log(ε/3.7D + 2.51/Re√f), applied for turbulent flow (Re > 4000).
For rectangular ducts, the equivalent circular diameter is: De = 1.30 × (a×b)^0.625 / (a+b)^0.25. This allows circular duct friction equations to be applied to rectangular sections.
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This duct sizing calculator helps HVAC engineers, mechanical designers, and contractors determine the correct duct dimensions for air distribution systems. Enter the airflow rate and duct dimensions to instantly calculate air velocity, friction pressure drop per metre, and total pressure drop for a given duct run. The calculator supports both rectangular and circular duct profiles with multiple material options including galvanised steel, stainless steel, aluminium, and fibreglass duct board.
The calculator uses the Darcy-Weisbach equation for pressure drop and solves the Colebrook-White equation iteratively to determine the friction factor based on the Reynolds number and pipe roughness. For rectangular ducts, the Huebscher equivalent diameter formula converts the rectangular cross-section into an equivalent circular diameter. These are the standard methods used in HVAC engineering practice and referenced throughout AIRAH and ASHRAE design guides.
This tool provides preliminary sizing results suitable for schematic design and feasibility checks. Final duct sizing for construction documentation should account for fitting losses, system effects, duct leakage, and comply with AS 4254 for ductwork construction. For complex systems with multiple branches, variable air volume control, or noise-sensitive applications, consult a qualified mechanical engineer.