Calculator Inputs
Enter design airflow, velocity limits, friction limits, length, and flexible duct correction factors.
Example Data Table
| Airflow | Target Friction | Max Velocity | Typical Size | Common Use |
|---|---|---|---|---|
| 100 CFM | 0.08 in/100ft | 700 FPM | 6 in | Small room branch |
| 250 CFM | 0.08 in/100ft | 700 FPM | 8 in | Medium branch run |
| 400 CFM | 0.08 in/100ft | 700 FPM | 10 in | Large supply branch |
| 700 CFM | 0.07 in/100ft | 750 FPM | 14 in | Main flexible section |
Formula Used
The calculator estimates round flexible duct size using airflow, velocity, and friction checks.
Velocity area:
Area = CFM / Velocity
Round diameter from velocity:
D = √((4 × Area × 144) / π)
Approximate duct friction rate:
FR = 0.109136 × CFM^1.9 / D^5.02
Corrected friction:
Corrected FR = Base FR × Compression Factor × Roughness Factor
Total pressure loss:
Total Loss = Corrected FR × Equivalent Length / 100
This is an estimating tool. Final HVAC design should follow local codes, manufacturer tables, and professional balancing practices.
How to Use This Calculator
- Enter the required airflow in CFM for the room or branch.
- Set the maximum velocity allowed for comfort and noise control.
- Enter the target friction rate for the duct run.
- Add straight duct length and equivalent fitting length.
- Adjust compression and roughness factors for flexible duct quality.
- Enter an allowed pressure loss for the run.
- Submit the form and review the recommended standard size.
- Use CSV or PDF export for project records.
Flexible Duct Sizing Guide
Why Flexible Duct Size Matters
Flexible duct sizing affects comfort, noise, energy use, and airflow balance. A duct that is too small can create high velocity. High velocity often increases noise near grilles. It can also increase pressure loss across the duct path. When pressure loss rises, the blower works harder. Rooms may receive less air than expected.
Airflow and Velocity
Airflow is usually entered in cubic feet per minute. The calculator compares that airflow with the selected velocity limit. Lower velocity can help quiet systems. Higher velocity can reduce duct size, but it may create more noise. Many comfort designs use moderate velocity for branch runs.
Friction and Equivalent Length
Friction rate is another key limit. It estimates how much pressure is lost per one hundred feet of duct. Flexible duct can lose more pressure than smooth metal duct. Compression, sagging, bends, and rough inner liners all increase resistance. That is why this tool includes correction factors. Equivalent fitting length converts bends and fittings into added duct length. This gives a better pressure loss estimate.
Correction Factors
Flexible duct performs best when it is fully stretched. Sharp bends should be avoided. Long unsupported sections should be reduced. A compression factor above one raises the friction estimate. A roughness factor also raises the estimate. These factors help represent real installation conditions.
Reading the Result
The recommended diameter is based on both velocity and friction. The calculator chooses the larger requirement, then applies a safety factor. It also rounds up to a standard duct size. If a manual diameter is entered, the tool checks that diameter instead. This is useful when reviewing an existing installation.
Practical Design Advice
Use the result as a planning guide. Confirm final sizes with manufacturer duct charts and local design rules. Check available fan static pressure before installation. Balance the system after startup. Good sizing, smooth routing, and proper support usually give better comfort and quieter airflow.
FAQs
1. What does this flexible duct sizing calculator do?
It estimates round flexible duct diameter from airflow, velocity, friction rate, length, and correction factors. It also checks velocity and pressure loss.
2. What is CFM?
CFM means cubic feet per minute. It shows the volume of air moving through the duct each minute.
3. Why does flexible duct need correction factors?
Flexible duct often has more resistance than smooth duct. Compression, bends, sagging, and rough liners can increase pressure loss.
4. What is a good velocity limit?
Many comfort branch ducts use moderate velocity limits. Lower values can reduce noise, while higher values may reduce duct size.
5. What is equivalent fitting length?
It converts elbows, bends, boots, and fittings into added duct length. This helps estimate total pressure loss more realistically.
6. Should I use the recommended standard size?
Usually, the next larger standard size is safer. It reduces velocity and helps lower pressure loss in the run.
7. Can this check an existing duct?
Yes. Enter the existing diameter in the manual diameter field. The result will check velocity and pressure loss for that size.
8. Is this enough for final HVAC design?
No. Use it for planning. Final design should follow codes, equipment data, duct charts, and professional field balancing.