Duct pressure drop using Darcy–Weisbach friction factor.
Rectangular friction via the Huebscher circular-equivalent duct. Fittings via K-factor (ΣK).
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About this duct pressure drop calculator
This free duct pressure drop calculator works out the total pressure loss along a run of ventilation ductwork, combining straight-duct friction with the loss across bends, tees, dampers, grilles and other fittings. Enter the airflow (in m³/s, L/s or m³/h), the duct length, the shape and size, rectangular width and height, or a circular diameter, then pick a surface roughness preset (galvanised steel, mild steel, stainless/aluminium, smooth plastic or concrete) and adjust air temperature and pressure if needed. It is built for UK building-services engineers, ductwork designers and HVAC contractors sizing and balancing low-velocity systems.
How the pressure drop is calculated
Friction loss uses the Darcy–Weisbach equation with the friction factor from the Swamee–Jain correlation (and 64/Re in the laminar region). Rectangular ducts are reduced to the ASHRAE Huebscher circular-equivalent diameter, De = 1.30·(wh)0.625/(w+h)0.25, so friction is computed on the equivalent round duct carrying the same flow. Air density comes from the ideal gas law and viscosity from Sutherland’s formula. Fitting losses are summed as ΣK × the dynamic pressure (½ρv²). The tool reports velocity, Reynolds number, flow regime, Mach number and friction rate, with advisory notes referenced to CIBSE Guide B low-velocity bands (3–6 m/s branches, 5–8 m/s mains).
Frequently asked questions
Does it handle both rectangular and circular ducts?
Yes. Choose rectangular and enter width and height, or circular and enter diameter. Rectangular friction is computed on the Huebscher circular-equivalent duct, while the displayed velocity and Reynolds number use the duct’s actual cross-section and hydraulic diameter.
How are fitting losses included?
Add fittings from the built-in K-factor library (bends, tees, transitions, dampers, silencers, grilles, filters and more), each with a quantity and optional K override. Their effective values are summed as ΣK and multiplied by the dynamic pressure. You can also enter a manual ΣK instead.
What velocity should I aim for?
The advisory notes follow CIBSE Guide B low-velocity guidance, around 3–6 m/s for branch ducts and near terminals, and 5–8 m/s for main ducts. Higher velocities trigger warnings about acoustics, pressure class and fan energy. Results are for design guidance only; always verify with a qualified engineer.