Fabric Duct Sizing Calculator

We recommend sizing fabric ducts and diffusers by velocity or the air speed in the duct. If the velocity in the duct is too high, the negative consequences can be excessive noise, duct movement (Vibration) from turbulent air / rapid pressure fluctuations and ultimately, reduced longevity.

Design best practices for metal systems upstream of  textile diffusers should be applied: match the inlet velocity of the fabric duct, incorporate 3X diameter metal duct ahead of the fabric whenever possible, and design elbows with 1.5X diameter radius or turning vanes.

Contact us for applications where these conditions are not possible.

For round duct socks try to stay below 1500fpm velocity if possible. Always practice good metal duct layout prior to the fabric inlet: match the velocity of the fabric duct, use 3x diameter of straight metal duct if possible and always try to use 1.5X diameter radius elbows or elbows with turning vanes.


Velocity recommendations:

>800 top - Top inlet half round

700-1000 fpm - Sound sensitive (please contact us for these selections)

1200-1400 - Where fittings are present or if half round duct

1400-1600 - Straight round duct runs

1600+ Not recommended (please contact us for design assistance)


How to use the calculator

Choose your shape on the right-hand side.

Choose the units you’re most comfortable with M3/h or l/s or CFM.

Enter your volume of air and velocity (follow velocity guidelines above) to get the duct size*.


*Note - For half round diffusers you can have a round top inlet, or multiple top inlets. You would need to select the round for the top inlet sizing then select the half-round for the actual velocity in the length wise direction of the half round. Feel free to contact us for more complex designs, we are here to help!


Velocity slider colors

Red = Too much velocity (Consequences of higher velocity are vibration and lower duct longevity) Vibration

Green = Generally in spec for Prihoda Fabric Ducts and Diffusers

Blue = Possibly oversized unless sound sensitive application (800-1000fpm)

Straight Round Duct Example: You have an 8-ton unit with 4000cfm and you want a velocity of 1500FPM in a straight round duct.

  1. Choose round shape on the right.
  2. Input 1,500CFM in the air volume box and then adjust the velocity to 1500FPM with the slider or text box
  3. Looking at the diameter it’s suggesting 23 inches.  As everything is made to order we can make a 23inch duct or you can round up. Enter 24inch into the duct size box for a common duct size and you’ll see that it moves the velocity to 1300FPM.
  4. Relax, you just perfectly sized a fabric duct diffuser. Well done!


Please note the calculator is for guidance only. We do not take responsibility for your duct selections or sizes using this software; it’s for guidance only and we would always recommend you discuss specific selections with us. Contact us for applications where these conditions are not possible.

Have fun!!





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