Fan Dilution System Duct Size Calculator

Use the calculator below to determine the most appropriate circular duct size to use when calculating fan dilution system requirements.

Please note that Java must be enabled on your browser in order to use this calculator. Java is available from here.

The volumetric flow rate in in fan dilution systems is expressed as:

Volumetric flow rate in fan dilution systems equation


  • Q is the volumetric flow rate
  • Fn is the fan dilution factor
Fan dilution systems are typically designed and sized to provide a flow velocity of between 6 and 8 m/s. The calculator above aims to select the most appropriate duct from a selection of commonly available and stocked duct sizes.

The duct sizes available are 80, 100, 125, 150, 160, 180, 200, 224, 250, 280, 300, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250, 1320, 1422, 1524, 1600.


The software provided herein is provided free of charge and may be freely distributed so long as this disclaimer remains in place. Blaney Mechanical Ltd provide this software as is and do not express or imply any warranty of merchantibility nor any warranty of fitness for a particular purpose. If you wish to use this software, you should satisfy yourself that it is suitable for the intended purpose.

30 Day Credit Accounts Available Subject to Status