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Enter flow rate (L/s) and duct diameter (mm) to begin sizing



  1. Select the duct material from the drop-down list (most duct sizing tools are typically based on galvanised steel ducting by default)

  2. Enter the air flowrate in litres/second (can change to cubic metres/second by use of drop-down to the right of the input)

  3. Enter an internal duct diameter (this can be any number, it doesn’t need to be standard duct size)

  4. To get an idea of the equivalent rectangular duct size, adjust the aspect ratio. Note that only aspect ratio up to 5:1 are selectable, as above this the results are not reliable.

  5. The resulting velocity and pressure drop per unit length are displayed in the blue box.

Note that there is no option for flexible duct. This is due to the large number of variables that can effect the pressure drop for flex, such as whether it is pulled taut or loose, straight or with bends. In practice, it is not uncommon for flexible duct to have up to around 5 or 6 times the resistance of the equivalent sized straight galvanised steel duct. Therefore the designer should make suitable allowance for flexible duct based on their knowledge of the system arrangement.