Flex Duct CFM Chart: Every Size from 4″ to 18″ (2026)
How much air each flex duct size actually carries, the friction-rate math behind it, and the sizing mistakes that choke airflow.
If you need the number fast, it's in the table below. If you need to know why — and when flex duct quietly under-delivers versus rigid — keep reading. Every value here follows ACCA Manual D flex-duct figures at the industry-standard 0.10 in. w.g. friction rate, fully extended (not compressed) installation.
Flex Duct CFM Chart (0.10 in. w.g. friction rate, fully extended)
| Flex Duct Diameter | Max Recommended CFM | Typical Use |
|---|---|---|
| 4″ | 20–25 CFM | Bath exhaust (small), toe-kick runs |
| 5″ | 45–50 CFM | Half-bath, small laundry exhaust |
| 6″ | 75–80 CFM | Small bedroom / office supply |
| 7″ | 110–120 CFM | Standard bedroom supply |
| 8″ | 160–170 CFM | Large bedroom, small living room |
| 9″ | 225–230 CFM | Living room, master suite supply |
| 10″ | 300–325 CFM | Great room, small trunk branch |
| 12″ | 500–525 CFM | Branch trunk, open-plan zones |
| 14″ | 700–750 CFM | Main trunk (2–2.5 ton systems) |
| 16″ | 975–1,000 CFM | Main trunk (3–3.5 ton systems) |
| 18″ | 1,300–1,400 CFM | Main trunk (4–5 ton systems) |
Why flex duct carries less air than you think
The numbers above assume a perfect install: pulled tight, gentle turns, properly supported. Flex duct's corrugated inner liner has roughly 2–3× the friction of smooth rigid pipe at the same diameter. Real-world penalties stack fast:
- Compressed or sagging runs: a flex run installed at 85% extension can lose 15–40% of its rated CFM. Support every 4 feet, sag no more than 1/2″ per foot.
- Sharp bends: every 90° kink acts like several extra feet of duct. Keep bend radius at least one duct diameter.
- Long runs: keep flex runs under ~25 feet. Beyond that, upsize or switch to rigid.
- Crushed sections: common in attics after other trades step through. One crushed 8″ run can drop a bedroom from 160 CFM to under 100.
Flex vs. rigid round at the same size
| Diameter | Flex CFM (0.10 FR) | Rigid Round CFM (0.10 FR) | Flex Penalty |
|---|---|---|---|
| 6″ | 75–80 | ~105 | ~25–30% |
| 8″ | 160–170 | ~230 | ~28% |
| 10″ | 300–325 | ~420 | ~26% |
| 12″ | 500–525 | ~680 | ~24% |
If a design calls for more than ~100 CFM on a long run, rigid round (or a short rigid trunk with flex drops) almost always wins on delivered airflow — even though flex wins on install speed and cost.
Working backward: room CFM first, duct size second
Never size duct from the chart alone. Size the room load first, then pick the diameter that delivers it:
| Room Type | Typical CFM Need | Flex Size That Delivers It |
|---|---|---|
| Small bedroom (100–150 sq ft) | 50–80 | 6″–7″ |
| Master bedroom | 100–150 | 7″–8″ |
| Living room | 140–230 | 8″–9″ |
| Great room / open plan | 250–400 | 10″–12″ (or two 8″ drops) |
| Bathroom (exhaust) | 50–110 | 5″–7″ |
Our full HVAC Duct Sizing Chart covers return sizing, rectangular equivalents, and total-system balance; the duct sizing calculator does the friction-rate math for your exact run lengths.
The three failures we see on job sites
1. Undersized returns
Supply gets all the attention; returns strangle the system. A 3-ton system needs ~1,200 CFM returned — a single 14″ flex return (725 CFM max) is not enough. Split returns or upsize.
2. Flex used as long trunk
Flex is a branch material. A 40-foot flex trunk at 4% compression is why the far bedrooms never cool. Rigid trunk, flex drops.
3. Sizing by tonnage alone
"3-ton house = 12″ branches" ignores room loads, window gain, and ceiling height. Manual J (load) → Manual S (equipment) → Manual D (ducts). In that order, every time.
Ducting a job this week?
We stock flex duct, rigid round, takeoffs, boots, and the mastic to seal it — contractor pricing, same-week freight.
Get a Quote →Flex duct CFM — quick answers
How many CFM does a 12-inch flex duct carry?
About 500–525 CFM at the standard 0.10 in. w.g. friction rate, fully extended. Compressed or sagging installs can lose a third of that.
How many CFM does a 6-inch flex duct carry?
75–80 CFM — enough for a small bedroom or office supply run.
Is rigid duct better than flex?
For airflow, yes: rigid round carries roughly 25–30% more CFM at the same diameter and friction rate. Flex wins on install speed and cost for short branch drops.
What is the maximum length for a flex duct run?
Keep branch runs under about 25 feet. Longer runs compound friction loss; upsize the duct or switch to rigid.
How much CFM per ton of cooling?
Design target is ~400 CFM per ton. A 2.5-ton system needs ~1,000 CFM total; a 4-ton needs ~1,600 CFM.
Portlandia Electric Supply · Technical Content Team · Data follows ACCA Manual D flex-duct friction charts at 0.10 in. w.g.; field-verify against your local code and equipment manual.
