Fabric Duct Velocity Calculator: engineer's guide
Fabric ducts distribute air through fabric permeability, orifices, or nozzles. Inlet velocity must match the distribution method for even throw.
Open the Fabric Duct Velocity calculator →
What this calculator does
The calculator converts CFM and duct diameter to inlet velocity and compares against the manufacturer's recommended range for the selected distribution method.
Formulas used
- Inlet velocity:
V = CFM / A = CFM × 576 / (π D2)
Variables
- D = duct diameter, inches
- A = cross-section area, ft²
Worked example
Scenario: 4000 CFM into a 24 in fabric duct.
- A = π × 242 / (4 × 144) ≈ 3.14 ft²
- V = 4000 / 3.14 ≈ 1273 fpm
- For permeable-fabric: within 1000 to 1400 fpm target range
- For orifice distribution: raise velocity to 1500 fpm range
Common mistakes to avoid
- Same velocity for permeable and orifice distribution
- Sock ballooning at high velocity or collapse at low velocity
- Missing tension cables for long runs
- Ignoring room air stratification for high-ceiling spaces
Frequently asked questions
Fabric duct benefits?
Low weight, even distribution, no condensation on the outside, easy to wash.
Where not to use?
Aggressive contaminant zones (labs), or spaces requiring rigid smoke control ductwork.
Standards and references
AMCA 20, ASHRAE Applications Ch. 18
Related tools on this site
Open the calculator that pairs with this guide: Fabric Duct Velocity Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.