Flex Duct Airflow Calculator
Pick the picture that looks most like your ductwork. We'll estimate how much airflow your system is losing before it ever reaches the room — compared to smooth-walled rigid pipe at the same length and fan pressure.
Look at any exposed run in your attic, crawlspace, or closet. Pick the option that's closest — it doesn't have to be perfect.
Not sure which one? Look for the worst section on the run that feeds your problem room — the one that's too hot, too cold, or gets weak airflow.
Model a single flex-duct run against smooth-walled rigid pipe at the same conditions. Adjust the sliders and watch the delivered airflow update live.
Reynolds, friction, and delivered airflow at your current geometry.
System curves at your current geometry. The dashed line marks your fan static pressure; where each curve crosses it is the delivered CFM. Drag or hover along the chart.
Reynolds and friction factor via Haaland approximation to Colebrook-White at 70°F standard air. Flex roughness scales with compression per ASHRAE Fundamentals Ch. 21. Compression penalty follows ACCA Manual D and Energy Vanguard field data.
Compared to smooth-walled rigid pipe at the same length and fan pressure.
Reynolds and friction factor computed via Haaland approximation to Colebrook-White at 70°F standard air. Roughness scales with compression per ASHRAE Fundamentals Ch. 21. Compression penalty follows ACCA Manual D and Energy Vanguard field data.
This is a bounded estimate. The model compares one duct run against smooth-walled rigid pipe under the same modeled conditions. It doesn't include duct leakage, register/boot losses, filter or coil pressure drop, or other branches. Numbers on a screen are a hypothesis — measurement settles it.
Same 8″ round duct, same 25 ft run, same 0.10″ WC fan static pressure. The only thing that changes is how the flex is installed. Your current selection is highlighted.
| Install condition | CFM | Loss |
|---|---|---|
| Rigid metal pipe (reference) | 423 | — |
| Flex, pulled taut (best practice) | 363 | −14% |
| Flex, 4% compression | 354 | −16% |
| Flex, 15% compression | 290 | −31% |
| Flex, 30% compression + tight bend | 217 | −49% |
| Flex, 45% sag + kinked turn | 156 | −63% |
Sources: ACCA · Energy Vanguard · OSTI · ASHRAE Fundamentals Ch. 21 · Darcy–Weisbach / Reynolds.
Each install condition plotted against airflow loss. The curve steepens quickly — small install defects have big airflow consequences.
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