Flex Duct Derating Calculator
How much airflow compressed flexible duct really delivers versus rigid metal.
Fully stretched flex is nearly as good as metal; the helix exposed by compression is catastrophic. Pull it taut and strap every 1.2 m — free CFM.
Formula
Note: Multiplier is an empirical fit to published test data; actual loss depends on sag and bends.
Flex Duct Derating Calculator is a free flex duct friction for HVAC designers, technicians and energy auditors — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Flex Duct Derating Calculator
How much airflow compressed flexible duct really delivers versus rigid metal. The calculation implements Friction ×4 at 15% compression, ×9 at 30% (Texas A&M data); Q ∝ 1/√(f-mult) (Texas A&M / ACCA flexible duct research). Fully stretched flex is nearly as good as metal; the helix exposed by compression is catastrophic. Pull it taut and strap every 1.2 m — free CFM.
How to use Flex Duct Derating Calculator
- 1Enter Design airflow (as if rigid) in L/s.
- 2Enter Linear compression in % (Sagging, unstretched flex easily hits 15–30%).
- 3Read Friction multiplier vs rigid, Realistic delivered flow instantly — no submit button needed.
- 4Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Flex Duct Derating Calculator?
- ✓Implements the standard formula — Friction ×4 at 15% compression, ×9 at 30% (Texas A&M data); Q ∝ 1/√(f-mult)
- ✓Reference cited on-page: Texas A&M / ACCA flexible duct research
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Flex Duct Derating Calculator use?+
It computes Friction ×4 at 15% compression, ×9 at 30% (Texas A&M data); Q ∝ 1/√(f-mult), per Texas A&M / ACCA flexible duct research. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
Fully stretched flex is nearly as good as metal; the helix exposed by compression is catastrophic. Pull it taut and strap every 1.2 m — free CFM. Note: Multiplier is an empirical fit to published test data; actual loss depends on sag and bends.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Flex Duct Derating Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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