Ductwork Sizing Calculator — HVAC Duct Dimensions

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.

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.

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 has by far the highest friction. Fully extended flexible duct (modelled here at ~3.0mm roughness, within ASHRAE’s 1.0–4.6mm rough-class range) has roughly twice the friction of rigid galvanised steel; but compressed or sagging flexible duct is far worse — ASHRAE Fundamentals shows correction factors making it several times higher again. Keep flexible duct fully stretched, supported, and as short as possible — ideally under 1.5m per CIBSE Guide B.

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 rounds each dimension UP to the next 25mm increment, so the actual cross-section is never smaller than required and the actual velocity stays at or below your design velocity.

Each bend, tee, reducer, or damper adds to the total system pressure drop. Bend loss is the loss coefficient K multiplied by the velocity pressure (0.5 × rho × v²). Our calculator uses K = 0.5 as an indicative default for a medium-radius 90-degree bend, but the real figure depends on the fitting: a sharp mitred elbow is roughly K = 1.0–1.3, while a smooth long-radius bend can be as low as K = 0.2. The total system pressure drop determines the fan size needed. For full system design, take the exact K values for every fitting from CIBSE Guide C (or ASHRAE Fundamentals) fitting loss tables.