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Professional calculator for sizing ventilation duct cross-sections in mechanical ventilation systems. Linear pressure losses are computed using the Darcy-Weisbach equation, with absolute roughness chosen by duct type (Spiro k = 0.10 mm, galvanised steel k = 0.15 mm, plastic k = 0.03 mm, flex from 0.75 to 2.0 mm). The calculator covers both round ducts (Spiro, plastic) and rectangular ducts, returning a complete list of available sections together with the resulting air velocity and unit pressure drop for every airflow. A tool for designers of ventilation, air-conditioning and heat-recovery systems.
Enter the airflow in m³/h or dm³/s — the calculator converts between the two automatically. This is the value you would normally take from a room ventilation balance (kitchen, bathroom, living room) or from the relevant national standard.
Choose the duct type — round Spiro or plastic for round ducts, or use the dedicated rectangular duct calculator. The duct type drives the absolute roughness and therefore the linear pressure losses.
Read the results: a full table of diameters (or width × height for rectangular) with air velocity v [m/s] and unit pressure drop Δp [Pa/m]. Pick the section that meets the velocity criterion and the pressure losses acceptable in your system.
Round ducts (Spiro) — hydraulically the most efficient, they give the lowest pressure losses for a given flow and are easy to insulate. The default choice for residential ventilation, heat recovery, and anywhere there is enough room above a suspended ceiling or in a shaft. Lower unit material cost.
Rectangular ducts — chosen when limited build-up height forces a flat cross-section (e.g. above a suspended ceiling in a corridor or in retail buildings). They have higher pressure losses than a round duct of the same cross-sectional area and require more sealing.
Hydraulic diameter — for a rectangular duct with sides a × b it is D_h = 2ab/(a+b); the calculator uses it to determine the Reynolds number, the friction factor and the pressure drop. For an accurate comparison of a rectangular duct with a round one at equal pressure loss, the equivalent diameter per Huebscher is used. The aspect ratio a:b should not exceed 1:4 and is best kept within 1:2.
Sizing ventilation ducts is driven primarily by the velocity criterion — keeping the velocity in ducts and fittings within the right band determines acoustic comfort, acceptable pressure losses and reasonable investment cost at the same time. The values below follow common Polish design practice and the Recknagel handbook:
The starting point is the air flow rate and the permissible flow velocity. From A = V̇ / w you derive the required cross-sectional area, then select the nearest duct from the standard range. For a given flow the calculator shows the list of available cross-sections together with the velocity and the specific pressure drop (Darcy-Weisbach method), so the trade-off between noise, resistance and duct size is immediately visible.
For comfort ventilation (homes, offices, schools) main ducts are designed for 3–6 m/s, and branches near diffusers for 3–4 m/s for quiet operation. The fresh-air intake — max. 2.5 m/s, the exhaust — max. 4 m/s. Industrial ventilation and extraction systems run at 6–10 m/s and more; for dust transport, minimum transport velocities apply (from around a dozen up to about 25 m/s).
Round ducts are the most hydraulically efficient — they give the lowest pressure loss for a given flow, are cheaper and easier to insulate; they are the standard for residential ventilation and heat recovery. Rectangular ducts are chosen when limited construction height forces a flattened cross-section (e.g. above a suspended ceiling); they have higher resistance and require more sealing.
Velocities above 6–7 m/s in comfort ducts generate hydrodynamic noise and sound regeneration at tees and reducers, and also increase resistance and fan energy use. In rooms with elevated acoustic requirements (bedrooms, conference rooms) correspondingly lower velocities are adopted — down to 3–5 m/s.
The equivalent diameter is the diameter of a round duct that gives the same specific pressure drop at the same air flow — it is found from Huebscher's formula d_e = 1.3 × (a·b)^0.625 / (a+b)^0.25 and is used to compare rectangular ducts with round ones. It should be distinguished from the hydraulic diameter D_h = 2ab/(a+b), which describes the flow geometry and is used to determine the Reynolds number and the friction factor λ.
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