A flex duct calculator sizes flexible (insulated) duct the same way as rigid metal — CFM plus friction rate — but with a critical difference: flex carries less air at the same nominal diameter. A flexible duct calculator must derate capacity or upsize one increment. A round duct calculator for rigid metal does not apply directly to flex without adjustment.
Flex vs Rigid Capacity
| Nominal size | Rigid metal CFM (~0.10 fr) | Flex duct CFM (derated) |
|---|---|---|
| 6 in | 100 | 80 |
| 8 in | 225 | 180 |
| 10 in | 450 | 360 |
| 12 in | 700 | 560 |
| 14 in | 1,100 | 880 |
Flex duct vs rigid metal CFM capacity
Flex duct carries 15–20% less than rigid at the same diameter due to ribbed wall friction. Poorly stretched flex (sagged, compressed, sharp bends) can lose 50%+ of rated capacity.
Flex Duct Sizing Rules
- Stretch fully extended — compressed flex reduces effective diameter
- Limit length — ACCA Manual D recommends max 15 ft per flex run where possible
- Upsize one increment — if rigid chart says 8 in, use 10 in flex
- Avoid sharp bends — use long-sweep elbows; kinks choke airflow
- Support every 4 ft — sagging reduces diameter and increases friction
Five flex duct installation rules
Round Duct Calculator Reference
Rigid round duct calculator output at 0.10 in.wg/100 ft:
| CFM | Round size |
|---|---|
| 100 | 6 in |
| 225 | 8 in |
| 450 | 10 in |
| 700 | 12 in |
For flex, select the next size up from this round duct calculator table, or use our duct size calculator with flex derating.
Round duct to flex upsize reference
When Flex Is Appropriate
- Last few feet to register — connects rigid trunk to ceiling or floor boot
- Attic drops — where rigid routing is impractical
- Retrofit — adding a supply to an existing room
Avoid flex for long trunk runs, horizontal spans over 15 ft, or any run you cannot inspect and support properly.
See HVAC Duct Calculator Guide and Online Ductulator Guide for rigid duct sizing.
Use the Free Calculator
HVAC Duct Size Calculator — flex duct calculator and round duct calculator with friction rate control.
FAQ
How do I use a flex duct calculator?
Size for CFM at target friction rate, then upsize one diameter from the rigid-metal chart — or derate flex capacity by 15–20%.
What is the difference between flex duct calculator and round duct calculator?
Round duct calculator assumes smooth rigid metal. Flex duct calculator must account for higher friction from corrugated walls.
Can flexible duct replace rigid trunk lines?
Not recommended. Flex is for short branch connections. Long flex trunks sag, restrict airflow, and fail inspection in many jurisdictions.
What round duct size for 400 CFM in flex?
Rigid chart says 10 in for ~450 CFM. Use 12 in flex for 400 CFM to maintain acceptable velocity and friction.