Enter airflow, application and duct type to get the correct duct diameter, velocity and pressure drop.
ASHRAE recommends maximum duct velocities of 5–8 m/s for main ducts and 3–6 m/s for branch ducts depending on the application. Residential systems use lower velocities (3–5 m/s) to minimise noise. This calculator applies these limits automatically.
The equal-friction method sizes all duct sections to the same pressure drop per unit length (typically 0.8–1.2 Pa/m). This balances airflow across branches and simplifies design. This calculator uses Darcy-Weisbach to compute friction pressure drop.
For 200 L/s in a residential main duct (max 5 m/s), the required diameter is approximately 225 mm. The next standard size up is DN250, giving a velocity of about 4 m/s — well within limits. Enter your exact values in the calculator above.