Flex Duct CFM Chart — Capacity vs Rigid & the Compression Penalty

Flex duct capacity done honestly: published flex sizing tables disagree by 20–40% with no stated basis, so this chart is built from the laboratory data instead — LBNL's fully stretched flex measurements against the same rigid-duct physics as our duct sizing chart, and the measured penalty for letting flex compress. The short version: taut flex gives up 5–15% to metal; slack flex gives up half the airflow.

Fully stretched flex vs rigid metal — CFM at the same friction rate

LBNL / ASHRAE 120 lab data
Airflow at 0.05 and 0.10 in. w.g. per 100 ft for the duct sizes LBNL lab-tested, rounded to 5 CFM. Flex values from the published fully-stretched power-law fits; rigid values computed from the same Colebrook physics as the duct sizing chart. Taut, properly supported flex only.
Duct ØRigid @ 0.05Flex @ 0.05Rigid @ 0.10Flex @ 0.10
6"756511095
8"165160240225
10"300270435380
LBNL tested 6", 8", and 10" — the workhorse branch sizes. For other sizes, size as rigid from the duct sizing chart and step up one size for flex, then hold the installation rules below.

The compression penalty — what slack costs

LBNL + Texas A&M
Measured pressure-loss multiplier when flex duct is installed compressed instead of taut (range across the tested 6"–10" sizes, LBNL correction model), and the airflow that survives if the available static stays fixed.
Installed conditionCompressionPressure lossAirflow retained
Fully stretched (taut)0%1.0×100%
Slightly loose4%1.6–2.0×70–78%
Visibly slack15%3.4–4.8×45–54%
Heavily compressed30%5.9–8.6×34–41%
Measured, not modeled: LBNL clocked 6" flex at 8.2–9.3× the taut pressure drop at 28.6% compression, and Texas A&M measured joist-supported sagging runs at more than 10× the loss of rigid duct. The old ADC rule of thumb — double the friction at 15% compression, quadruple at 30% — understates what both labs measured.

Installation rules that keep the chart true

ADC / UL 181
The ADC Flexible Duct Performance & Installation Standards requirements the capacity numbers depend on.
RuleRequirement
ExtensionInstall fully extended — never in a compressed state, never with excess length coiled in
Support spacing4 ft max (ADC 5th ed.; the 4th ed. allowed 5 ft) — a connection to rigid duct or equipment counts as a support
Sag1/2" max per foot of support spacing
SaddleSupport material at least 1-1/2" wide; vertical runs strapped at 6 ft max
BendsCenterline radius at least one duct diameter
Listing & closuresDuct listed to UL 181; joints closed with UL 181B-listed tape (181B-FX), mastic (181B-M), or clamps (181B-C)
Support rules per the ADC Flexible Duct Performance & Installation Standards — the 5th edition tightened support spacing to 4 ft from the 4th edition's 5 ft; the sag limit is unchanged. Cite the edition your jurisdiction or spec references, and check the mechanical code's flex-duct sections and the manufacturer's listing.

Why the standard design chart can mislead

The LBNL work found ACCA Manual D's flex friction chart overpredicts the pressure drop of TAUT flex by about 21% — mildly conservative — but underpredicts any compressed condition by up to 73%. In other words, the industry design chart is only valid for duct pulled tight, and there is no paper correction generous enough to rescue a run that's lying slack on the joists. Design at taut values, then build taut: pull the core tight, trim the excess, and support it per the table above. Budget the whole system with the static pressure calculator and check velocities on the air velocity chart. Local mechanical code and the duct's UL 181 listing govern the installation.

Common questions

How many CFM can 6 inch flex duct handle?

About 95 CFM at a 0.10 in/100 ft friction rate — if it is pulled completely taut. The same 6" rigid metal duct carries about 110 CFM. Let that run sag to 15% compression and the measured pressure loss climbs 3–5×, cutting airflow roughly in half. The size isn't the story with flex; the installation is.

Does flex duct really reduce airflow compared to metal?

Fully stretched, the laboratory answer is "a little": LBNL measured taut flex within about 15% of rigid metal duct capacity at the same friction rate. Compressed, the answer is "dramatically": anything beyond about 4% compression measured over 4× the pressure loss, and sagging joist-supported runs exceeded 10× rigid. Taut flex is fine; slack flex is a system killer.

Why do published flex duct sizing charts disagree?

Because the wall of a flex duct doesn't have one roughness: ASHRAE's flex category spans a 4× roughness range depending on how taut the helix is, and at least three table variants circulate online that differ 20–40% at the same size with no stated basis. This page publishes only the lab-measured data (LBNL / Texas A&M, the research behind ACCA Manual D's flex appendix) instead of picking a variant.

How often does flex duct need to be supported?

Per the ADC standard: every 4 feet at most (the current 5th edition; the older 4th edition allowed 5 feet), with no more than 1/2" of sag per foot of support spacing, saddles at least 1-1/2" wide, and bends at no less than one duct diameter of centerline radius. A connection to rigid duct or equipment counts as a support point.

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