Size round and rectangular ductwork from the required airflow and target air velocity for heating, cooling and ventilation systems.
The calculator divides the airflow by the velocity to find the required cross-sectional area, then converts that into a round duct diameter and an equivalent square duct size. Keeping velocity in the recommended range avoids excessive noise and pressure loss.
For quiet residential systems aim for roughly 3–5 m/s (600–1000 fpm) in branch ducts and up to 6–7 m/s (1200–1400 fpm) in main trunks. Higher velocities save space but increase noise and fan power.
1 litre/second is about 2.12 CFM, and 1 m³/h equals roughly 0.59 CFM. Choose the unit system that matches your design figures and the calculator handles the conversion.
Round duct is the most efficient and easiest to seal. Rectangular duct fits into tight ceiling voids. This calculator gives you the round diameter plus the equivalent square size for comparison.
Yes. Undersized ducts force air to move faster, which creates whistling and rumble. Sizing the duct so velocity stays in the recommended range keeps the system quiet.
The same area calculation applies to both supply and extract ducting. Just enter the airflow for the duct you are sizing and your chosen design velocity.
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