Flex Duct vs Rigid Duct: CFM Capacity, Sizing Up, and the Cost of Sag

Duct materials · Flex vs rigid · Updated 2026-07-22

Fully extended flex duct carries roughly 16% less airflow than galvanized rigid duct of the same diameter at the same friction rate, because its corrugated inner liner is about ten times rougher than sheet metal: ASHRAE puts galvanized at 0.0003 ft absolute roughness and fully extended nonmetallic flex at 0.003 ft. Recovering that airflow takes a diameter increase of about 7%, so a 6 in. rigid branch needs about 6.4 in. of flex and a 20 in. rigid trunk needs about 21.4 in.

That is the best case. Roughness is the smallest of three separate penalties, and most charts blur it together with the other two. This guide separates them, quantifies each from published testing, and gives the install rules that keep the design number real.

Flex duct vs rigid duct CFM chart

The table solves Darcy-Weisbach with the Colebrook friction factor at a friction rate of 0.10 in. wg per 100 ft, standard air, using ASHRAE Fundamentals Chapter 21 roughness: 0.0003 ft for galvanized steel, the value the ASHRAE round-duct friction chart is drawn at, and 0.003 ft for fully extended nonmetallic flex. ASHRAE gives fabric-and-wire flex a range of roughness rather than one value, so treat these as the top of what flex can do.

Two things fall out. The percentage penalty is nearly flat across the range, which is why the widely quoted pairing of about 1,700 CFM for 20 in. flex against about 2,000 CFM for 20 in. metal holds up as a ratio even though those absolute figures come from a lower friction rate. And the part rules of thumb get wrong: the upsize is a percentage, not a fixed number of inches. Seven percent of a 6 in. branch is four tenths of an inch, so it stays 6 in. Seven percent of an 18 in. trunk is more than a full inch, so it genuinely needs the next nominal size.

Three separate penalties

• Liner roughness. Baked into the chart above. Unavoidable, modest, and the only one you can design around cleanly.
• Linear compression. The run is cut longer than the distance it spans, so the wire helix never pulls taut. This is the expensive one and it is entirely an install defect.
• Sag between supports. Straps too far apart, or a strap that pinches. Sag adds effective length and bend, and it is what turns a run that passed on install day into a weak register two years later.

What compression actually costs

The reference test is Static Pressure Losses in 6, 8, and 10-inch Non-Metallic Flexible Ducts by Weaver and Culp at Texas A&M, popularized by Energy Vanguard's "The Science of Sag." The Air Diffusion Council publishes derating multipliers for the same condition. They do not agree, and the disagreement is the useful part.

Implied airflow assumes fixed available static pressure, where flow falls as the inverse square root of the friction multiplier. Work a 6 in. branch designed for 100 CFM through it: rigid gives 110 CFM of headroom, fully extended flex gives 94 and is already borderline, and the same run pulled 4% short of taut delivers about 70. Go to 7 in. flex and you have 142 CFM extended, which still clears 100 at that 4% defect. That is the honest case for upsizing flex, and it is a hedge against installation, not against physics.

Switch the material, re-run the math. Ductulator's calculator takes rectangular, round, or flex as a duct type and re-solves Darcy-Weisbach with Colebrook for the material you picked, then returns eight valid sizes annotated with velocity and friction rate. No mental fudge factor, and the ASHRAE citation sits under the number. Learn more about Ductulator.

Install rules that protect the number

1. Cut to length, then pull taut: Measure the span and cut to it. Extra length has nowhere to go but into compression, and it cannot be fixed later without cutting.
2. Support at 4 ft maximum: The ADC Flexible Duct Performance and Installation Standard calls for supports at the manufacturer's spacing and no more than 4 ft apart, with straps at least 1.5 in. wide so they do not pinch the liner. The International Mechanical Code sets the same 4 ft ceiling for flexible duct (Section 603.10 in recent editions).
3. Hold sag to 1/2 in. per foot of span: ADC's limit. On a 4 ft span that is 2 in. of droop, and that is the ceiling, not the target.
4. Keep flex on short branches off a rigid trunk: Galvanized for trunks and long runs, flex for the last few feet. That is where flex earns its keep on offsets and boot alignment without adding much effective length.
5. No tight radius at the boot: A hard bend at the takeoff or register boot adds equivalent length fast. Sweep the turn and support the duct through it so the inside of the radius does not kink.

Sizing flex in Ductulator

Enter the target CFM, set your friction rate, and choose flex as the duct type. Ductulator re-runs the full Darcy-Weisbach solution with the Colebrook iterative friction factor rather than applying a flat correction, so the eight candidate sizes it returns are already flex numbers. Each carries its velocity against the five-band velocity classifier, so an upsized branch that drops below a usable throw velocity gets flagged instead of shipping. Drop the run into the multi-segment static pressure budget with its fittings and you can see whether the flex penalty fits the available pressure before anyone cuts metal.

Frequently asked questions

How much CFM can a 6 inch flex duct handle?

About 94 CFM at a friction rate of 0.10 in. wg per 100 ft when fully extended, against roughly 110 CFM for 6 in. rigid galvanized. Published flex tables commonly show 75 to 100 CFM because manufacturers assume a real-world install rather than a perfectly taut one.

Do you need to upsize flex duct?

To match rigid capacity you need about 7% more diameter, which usually means no change on small branches and one nominal size up on trunks 14 in. and larger. Many designers upsize anyway as insurance against installation compression, since Texas A&M measured a 6 in. duct at just 4% compression down to about 70 CFM from about 110 CFM for rigid.

Is flex duct worse than rigid duct?

Fully extended and properly supported, flex is close to rigid metal - the Texas A&M testing found essentially no difference at 0% compression. The problem is that flex is easy to install badly and rigid duct is not, so the practical gap is an installation gap more than a material gap.

How far apart should flex duct supports be?

No more than 4 ft, per the Air Diffusion Council Flexible Duct Performance and Installation Standard and Section 603.10 of the International Mechanical Code, using straps at least 1.5 in. wide. Sag between supports should not exceed 1/2 in. per foot of spacing. Local mechanical codes may be stricter, so check the adopted code in your jurisdiction.

Keep reading

• CFM to Duct Size Chart: What Size Duct Do I Need for 400 CFM?
• Friction Rate in HVAC: What It Is and How to Calculate It
• Duct Velocity Chart: Recommended FPM for Supply, Return, and Branch Ducts