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Duct Calculator

Sheet Metal Duct Calculator

  • Enter known values for Air Flow Rate (CFM) and Friction Loss per 100FT of Steel Duct (inches of water).
  • Read computed values for Duct Diameter (inches) and Velocity (FPM).

Flow Rate (CFM):

Friction Loss (in. of water):

Diameter (inches):

Velocity (FPM):


  • Enter known values for Air Flow Rate (CFM) and Duct Diameter (inches).
  • Read computed values for Friction Loss per 100FT of Steel Duct (inches of water) and Velocity.

Flow Rate (CFM):

Diameter (inches):

Friction Loss (in. of water):

Velocity (FPM):

Equivalent Round Duct Diameter for Rectangular Opening

  • Equivalent Diameter: Diameter of a round duct that equates to the same pressure loss as the corresponding rectangular duct.
  • Enter the Length (inches) and Width (inches) of the Rectangular Duct.
  • Read computed values for Equivalent Round Duct Diameter (inches).

Length (inches):

Width (inches):

Equiv. Diameter (inches):

Equivalent Round Duct Diameter for Flat Oval Opening

  • Equivalent Diameter: Diameter of a round duct that equates to the same pressure loss as the corresponding flat oval duct.
  • Enter the Long Axis (inches) and Short Axis (inches) of the Flat Oval Duct.
  • Read computed values for Equivalent Round Duct Diameter (inches).
  • This tool can be used to work backwards to convert a Round Duct Diameter to an Equivalent Flat Oval Opening based on job parameters.

Long Axis (inches):

Short Axis (inches):

Equiv. Diameter (inches):

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