Flex Duct Sizing Chart
Flexible duct requires larger diameters than rigid metal duct for the same airflow. The corrugated inner liner creates significantly more friction than smooth galvanized steel. This chart applies the ACCA Manual D recommendation of sizing flex duct 2 inches larger than the equivalent rigid metal size (conservative, worst-case flex installation).
Flex duct that is compressed, kinked, or sagging can lose 50–80% of its rated capacity. Always pull flex fully taut, support every 4 feet, and limit runs to 5–6 ft where possible per ACCA Manual D guidance (as of June 2026 — verify with current ACCA standards).
Flex Duct Sizing Chart vs. Metal Duct
Base friction rate: 0.08 in.wg/100ft. Metal sizes use the equal-friction formula (d = 0.6312 × (Q/√fr)^0.4). Flex sizes = metal size + 2 in. per ACCA Manual D recommendation.
| CFM | Rigid metal size | Flex duct size | Velocity in metal (ft/min) | Flex note |
|---|---|---|---|---|
| 50 | 6" | 8" | 255 | Branch duct |
| 75 | 6" | 8" | 382 | Branch duct |
| 100 | 7" | 9" | 374 | Branch duct |
| 125 | 8" | 10" | 358 | Sub-trunk / branch |
| 150 | 8" | 10" | 430 | Sub-trunk / branch |
| 175 | 9" | 12" | 396 | Sub-trunk / branch |
| 200 | 9" | 12" | 453 | Sub-trunk / branch |
| 250 | 10" | 12" | 458 | Sub-trunk / branch |
| 300 | 12" | 14" | 382 | Trunk — prefer metal |
| 400 | 12" | 14" | 509 | Trunk — prefer metal |
| 500 | 14" | 16" | 468 | Trunk — prefer metal |
| 600 | 14" | 16" | 561 | Trunk — prefer metal |
| 750 | 16" | 18" | 537 | Trunk — prefer metal |
| 900 | 16" | 18" | 645 | Trunk — prefer metal |
For trunk ducts carrying 600+ CFM, rigid metal duct is strongly preferred over flex.
Flex Duct Sizing Rules of Thumb
- Always upsize 1–2 inches vs. equivalent metal duct (this chart uses +2" — conservative).
- Maximum run length: 5–6 feet for branch ducts. Longer runs accumulate friction rapidly.
- Support every 4 feet to prevent sagging, which collapses the liner cross-section.
- Keep bends gradual. A tight 90° bend in flex duct can equal 10–15 equivalent feet of additional length.
- Avoid flex for trunks — use rigid metal sheet duct for main trunk lines carrying 600+ CFM.
- Do not compress. Cut flex to final length fully extended, then connect — never leave slack.
Related Tools
- Duct sizing calculator — enter CFM, get round or rectangular size
- Duct sizing chart — full CFM reference table for metal duct
- CFM calculator — room dimensions to CFM
- Return air duct sizing
Frequently Asked Questions
No. Flexible duct has a corrugated inner liner that significantly increases friction compared to smooth metal. ACCA Manual D recommends sizing flex duct 1–2 inches larger than the equivalent metal size. Using the same size as metal will result in insufficient airflow.
ACCA Manual D recommends limiting flex duct runs to no more than 5–6 feet wherever possible. Each foot of flex duct is equivalent to roughly 1.5–2.5 feet of rigid metal duct equivalent length, depending on how well it is stretched and supported. Long, unsupported flex runs dramatically reduce system performance.
Compressed or sagging flex duct creates extreme friction loss. A flex duct compressed to 50% of its length can have 4× the friction loss of the same duct fully extended. Always pull flex duct taut between supports before measuring and cutting.
Yes, but the same upsizing rules apply. Return air ducts typically carry higher CFM over shorter distances, so the friction loss of flex becomes proportionally more significant. For return trunks over 6 feet, rigid metal duct is strongly preferred.