Duct Calculator FAQ


The Duct Calculator App has been designed to provide quick, accurate sizing information for existing ducts and what-if scenarios.


Locking Dimensions:

  • The lock symbols to the right of the duct dimensions indicate if
    that particular dimension is locked when calculating new equivalent
    values when a dimension is updated.
  • The Diameter can be locked or unlocked.
  • The Width or Height can be locked, but not both.

Using the Sliders:

  • Move the sliders for Airflow, Duct Diameter, Width and Height up
    and down to match the desired duct size to calculate velocity and
    pressure drop for.
  • The Diameter, Width and Height will change such that the
    equivalent size is shown:

    • Changing the diameter will updated either the equivalent Width
      or the Height, depending on which dimension is unlocked.
    • Changing the Width or Height will update the equivalent
      Diameter if it is unlocked, or the other rectangular duct dimension of
      the Diameter size is locked.

Direct Entry:

  • The values for Airflow, Diameter, Width and Height can be
    directly edited in the text boxes to allow quick entry of exact numbers.
  • Invalid entries will generate an error message, and must be
    corrected prior to the calculations updating.


  • The values for the calculated Velocity for Round Duct and
    Rectangular Duct are shown, as is the Pressure Drop per Length value.


  • Sheet metal ductwork, with an absolute roughness of 0.0003 ft
    (0.00762 mm).
  • Air at standard temperature and pressure.

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