What is the maximum recommended air velocity in ductwork?

CIBSE Guide B recommends maximum velocities of 3-5 m/s for supply and extract branches in domestic and low-noise commercial applications, and up to 6-10 m/s for main ducts in commercial systems. Higher velocities generate more noise and increase pressure drop, so the design velocity depends on the application and acceptable noise levels. For bedrooms and quiet offices, aim for 3-4 m/s; for kitchens and plant rooms, up to 6 m/s is typically acceptable.

Should I use round or rectangular ductwork?

Round ductwork is more efficient aerodynamically, cheaper per metre, and has lower pressure drop for the same air volume. Rectangular ductwork is used where ceiling void height is limited, as it can be made much flatter. For example, a 300mm round duct carries the same air volume as roughly a 400mm x 200mm rectangular duct, but the rectangular duct is only 200mm high. Flat oval ducts offer a compromise between the two.

How does duct material affect pressure drop?

Duct material roughness directly affects friction and pressure drop. Galvanised steel (roughness ~0.15mm) is the industry standard with moderate friction. Plastic/PVC ductwork (~0.03mm) has the lowest friction and is ideal for condensation-prone areas. Flexible duct (~3.0mm roughness) has by far the highest friction — up to 10x more than rigid duct — and should be kept as short as possible, ideally under 1.5m per CIBSE Guide B.

What aspect ratio should I use for rectangular ductwork?

CIBSE recommends keeping the aspect ratio (width to height) of rectangular ducts below 4:1 to maintain efficiency. Ratios above 4:1 significantly increase surface area and friction losses. The ideal aspect ratio is 1:1 (square duct), but practical constraints often require 2:1 or 3:1. Our calculator lets you set the desired aspect ratio and provides the standard dimensions rounded to the nearest 25mm.

How do I account for fittings and bends in duct sizing?

Each bend, tee, reducer, or damper adds to the total system pressure drop. A standard 90-degree bend adds roughly 0.5 times the velocity pressure (0.5 x 0.5 x rho x v^2). The total system pressure drop determines the fan size needed. Our calculator includes bend losses in the total pressure drop calculation. For full system design, add losses for all fittings using CIBSE Guide C fitting loss coefficients.

Ductwork Sizing Calculator — HVAC Duct Dimensions

How We Calculate This

This calculator determines duct dimensions using the equal velocity method as described in CIBSE Guide C. Given an air flow rate and maximum design velocity, it calculates the minimum duct cross-sectional area required.

Calculation method

For round ducts: Diameter = sqrt(4Q / (pi x V)), where Q is the volume flow rate in m3/s and V is the design velocity in m/s. The result is rounded up to the next standard metric duct size.

For rectangular ducts: The cross-sectional area is calculated from the same flow rate and velocity, then split into width and height using the selected aspect ratio. Dimensions are rounded to the nearest 25mm increment.

Pressure drop

Pressure drop per metre is calculated using the Darcy-Weisbach equation with the Colebrook-White friction factor. This accounts for duct material roughness, air density at the specified temperature, and Reynolds number. Bend losses are added using a loss coefficient of 0.5 per 90-degree bend.

The total system pressure drop determines the static pressure rating required for the fan or air handling unit serving the ductwork system.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.