Flex Duct Sizing — Why You Should Size Up From Metal Duct

This calculator's friction equation is built on smooth galvanized sheet metal — the standard engineering reference for round duct friction charts. Flex duct is not smooth: even fully stretched, its ribbed inner core creates meaningfully more resistance to airflow than rigid metal at the same diameter. If you're running flex instead of metal, the size this calculator hands you needs an adjustment. Here's the practical guidance.

Why flex duct has more friction

Rigid galvanized duct has a smooth interior, so the friction equation's constants are calibrated to that surface. Flex duct's inner liner has a corrugated, ribbed structure (even in the "smooth-wall" varieties, the wire helix core still creates periodic resistance), which increases turbulence and pressure loss per foot compared to an equally sized smooth duct. Two other factors compound this in real installations: flex duct is rarely stretched to its full rated length, and any sag or gentle bowing between supports acts like a series of soft, continuous bends rather than a straight run.

Roughly how much more friction

Industry duct-design references (SMACNA and ACCA Manual D guidance) generally treat fully-extended flex duct as having noticeably higher friction than smooth metal at the same diameter — commonly cited in the range of 3 to 8 times the friction, with the low end representing flex pulled taut and fully extended and the high end representing flex that's compressed, sagging, or poorly supported. That's a wide range because installation quality dominates the result far more than the flex product itself does — the single biggest lever you control isn't which flex duct you buy, it's how well it's installed.

Practical sizing adjustment

This calculator doesn't have a separate flex-duct mode — it always solves the metal-duct friction equation. Two practical ways to compensate when you know a run will be flex:

Size up one full standard increment from what the calculator recommends for metal (for example, if it returns a 10" round duct, plan for 12" flex on that run). This is the simpler field rule and is conservative for most reasonably well-installed flex runs.
Target a tighter friction rate for flex runs specifically — for example, run the calculator at 0.05 in.wg/100 ft instead of the standard 0.1 for any duct segment you know will be flex, which pushes the recommended metal-equivalent size up and gives more headroom for the extra friction flex adds.

Either approach is an approximation, not a precise flex-duct friction calculation — for a load-critical or unusually long flex run, a proper Manual D worksheet with flex-specific friction factors (or a contractor's own duct-sizing software with a dedicated flex mode) will be more accurate than either shortcut.

Installation matters more than the math

Because the 3–8× range above is driven mostly by installation quality, the highest-value fix for a noisy or underperforming flex run is often not resizing at all, but correcting how it's installed:

Pull it fully taut — even a small amount of slack meaningfully increases friction versus the fully extended state the better end of that range assumes.
Support it properly — hangers spaced closely enough (commonly every 4–5 feet, check manufacturer instructions and local code) to prevent sagging between supports.
Avoid tight bends — a sharp kink in flex duct can act like a near-total restriction at that point, far worse than the gentle equivalent-length penalty a smooth radius bend adds in rigid metal.
Keep runs short — flex is commonly used for the last connection from a rigid trunk to a boot or diffuser, not as a long primary run; the friction penalty compounds with length just like rigid duct's does, but from a higher starting point.

When metal is worth it instead

For long runs, high-CFM trunks, or anywhere noise is a priority (a run passing directly over a bedroom, for instance), rigid galvanized duct — sized directly from this calculator's default output with no adjustment — avoids the flex friction penalty and the installation-quality risk entirely. Flex duct's real advantage is fast installation and easy routing around obstacles on shorter branch runs, not lower cost-per-CFM performance.

FAQs

Does "smooth-wall" flex duct avoid this entirely? It reduces the penalty compared to standard wire-helix flex, but doesn't eliminate it — smooth-wall flex still typically has somewhat higher friction than rigid metal at the same diameter, and installation quality (tautness, support spacing, bend radius) still matters.

Can I just always size flex two increments up to be safe? That's conservative but not free — oversizing flex duct too aggressively can lead to poor air mixing at very low velocities and wastes material. One increment up, paired with a properly taut and supported installation, is the more common professional starting point.

Where do I check the metal-duct baseline size first? Start with the calculator or the duct sizing chart for your CFM, then apply the flex adjustment on top of that baseline.