How to read this chart
Pick the friction rate you're designing to, 0.08, 0.10, or 0.15 in.wc per 100 ft, and read down that column to find the largest CFM each nominal flex diameter can carry before it exceeds that friction rate. The paired fpm column shows the resulting air velocity at that CFM, so you can check it against a noise-driven velocity ceiling at the same time. For a specific run, use the calculator above: enter the actual CFM the run needs to carry and it returns the smallest nominal size that clears both your friction target and, if you set one, your velocity cap.
Why flex duct is not sized like rigid metal duct
Every number on this page comes from the Darcy-Weisbach equation, the same physics used to size any duct, pipe, or channel carrying a fluid. What changes between flex and rigid duct is a single input: surface roughness. Rigid galvanized steel duct has a roughness of roughly 0.09 mm, smooth enough that air slides past it with relatively little drag. Flexible duct's fibrous, corrugated inner liner is far rougher, roughly 0.9 mm in this calculator, and that rougher surface pulls more energy out of the airflow for every foot of travel. The result is that a flex duct run needs a larger diameter than a rigid duct run to hit the same friction rate at the same CFM, typically one or two nominal sizes larger. Charts that just relabel a rigid-duct table as "flex" are quietly wrong for exactly this reason: they use the wrong roughness, or none at all.
A friction-rate number on any chart, including this one, assumes the duct is fully extended and running in a reasonably straight line. Real installations often aren't. ACCA guidance notes that roughly 15% longitudinal compression, duct pulled tight enough to bunch the corrugations, can roughly double the effective friction rate compared to a fully stretched run. Sagging between supports, tight radius bends, and unnecessary length all do the same thing to a lesser degree. That means a run sized at 0.10 in.wc/100 ft on paper can behave like a 0.20+ run in the field if it's poorly installed, which shows up as weak airflow at the register, more noise, and a system that never quite performs like the load calculation predicted. Treat the 0.15 column here as a rough stand-in for a compressed or sagging install, not as a design target: the right fix for a run that would need 0.15+ is better support and a straighter path, not accepting the higher friction rate as normal.
Friction rate: which value to actually use
0.10 in.wc per 100 ft is the most common residential design default and a reasonable starting point if you have no other guidance from a load calculation (e.g., Manual D). 0.08 trades a slightly larger, quieter duct for lower velocity, useful on branches feeding rooms sensitive to register noise. Don't design to 0.15; use it only to understand how much margin you're losing if a run can't be kept straight and supported. Whatever rate you pick, apply it consistently across a system, mixing friction-rate assumptions between runs on the same trunk is a common source of an imbalanced system where some rooms are starved and others overpressured.
Velocity limits, and why they're separate from friction rate
Friction rate controls how much fan pressure a run consumes; velocity controls how loud it is. A common design ceiling is about 900 feet per minute for supply duct and 700 feet per minute for return duct, past that, register and duct noise become noticeable in a quiet room. It's possible to satisfy a friction-rate target and still exceed a velocity limit, or vice versa, which is why this calculator reports both and lets you optionally cap velocity when sizing a specific run.
How these numbers are derived
Air velocity comes from CFM divided by the duct's cross-sectional area. Reynolds number comes from that velocity, the diameter, and standard air's density and viscosity. The Darcy friction factor comes from the Swamee-Jain equation, an explicit, non-iterative approximation of the Colebrook-White equation accurate to within about 1% across the turbulent flow range this calculator operates in. Pressure loss per foot comes from the Darcy-Weisbach equation using that friction factor. Every step uses standard air at 20°C, sea level (ρ = 1.204 kg/m³, μ = 1.825×10⁻⁵ Pa·s); a different altitude or air temperature will shift the numbers slightly. Nothing here is transcribed from a manufacturer's chart or a scanned table, it's computed from the same public equations every duct designer already relies on, run live in your browser so you can verify the method rather than just trust the output.
Frequently asked questions
How many CFM can a 6 inch flex duct handle?
At a typical 0.10 in.wc/100 ft friction rate, a 6 inch flex duct (fully stretched) handles roughly 90-100 CFM before the friction rate or the 900 fpm supply velocity limit is exceeded. Check the table above for the exact figure at your chosen friction rate; a compressed or sagging installation handles noticeably less.
Is flex duct sizing different from metal duct sizing?
Yes. Flex duct liner is rougher than smooth galvanized steel, so it loses more pressure per foot at the same diameter and airflow. This calculator uses a flex roughness of 0.9 mm versus about 0.09 mm for galvanized, which is why flex duct typically needs to be sized one or two nominal sizes larger than rigid metal duct for the same CFM.
What friction rate should I use for flex duct?
0.10 in.wc per 100 ft is the common design default for residential supply runs. Use 0.08 for quieter, lower-velocity branches, and treat 0.15+ as the number a compressed or poorly-supported run will actually behave like, not a target to design toward.
Why is my flex duct louder or weaker than expected?
The most common cause is installation, not sizing. Flex duct that is compressed, sagging between supports, or run with tight bends behaves like a much smaller, rougher duct. ACCA guidance notes that around 15% longitudinal compression can roughly double the effective friction rate, which shows up as reduced airflow and higher noise at the register.
What velocity limit should supply and return flex duct stay under?
A common design ceiling is about 900 feet per minute for supply duct and 700 feet per minute for return duct, mainly to control noise. This calculator flags velocity alongside friction rate so you can check both at once.