Formula Used
The calculator uses the Darcy-Weisbach pressure loss relation with an added flexible duct stretch multiplier and bend loss term.
A = πD² / 4
V = Q / A
ρ = P / RT
Re = ρVD / μ
ΔP friction = f × (L / D) × (ρV² / 2) × C
ΔP bends = N × K × (ρV² / 2)
ΔP total = (ΔP friction + ΔP bends) × safety factor
For laminar flow, f = 64 / Re. For turbulent flow, the Swamee-Jain approximation estimates the Darcy friction factor. The compression multiplier rises when the flexible duct is not fully stretched.
How to Use This Calculator
- Enter the design airflow rate and select its unit.
- Enter the inside diameter of the flexible duct.
- Add straight duct length and any extra equivalent length.
- Enter air temperature, pressure, roughness, stretch, and bend data.
- Press the calculate button.
- Review total loss, friction rate, velocity, and Reynolds number.
- Use CSV or PDF export for records and design notes.
Example Data Table
| Airflow |
Diameter |
Length |
Stretch |
Bends |
Typical Use |
| 250 CFM |
8 in |
25 ft |
95% |
1 |
Small branch duct |
| 500 CFM |
10 in |
35 ft |
90% |
2 |
Medium flexible run |
| 850 CFM |
14 in |
45 ft |
85% |
3 |
Large supply path |
| 1200 CFM |
16 in |
60 ft |
80% |
4 |
Main flexible connection |
Flexible Duct Air Friction Loss Guide
Why Friction Loss Matters
Flexible duct moves air through curved and ribbed passages. That shape creates more resistance than smooth metal duct. The fan must overcome this resistance. If the loss is too high, rooms may receive less air. Noise can rise. Energy use can also increase. A good estimate helps the designer choose better duct size and routing.
Important Design Inputs
Airflow is the first input. Higher airflow raises velocity. Higher velocity raises pressure loss quickly. Diameter is also critical. A small duct can create large loss. Length adds more surface contact. Bends add local turbulence. Roughness represents the inner duct surface. A flexible liner usually has higher roughness than a smooth duct.
Effect of Stretch and Compression
Flexible duct should be pulled tight during installation. A compressed run has deeper folds. These folds disturb the air path. They increase friction and can lower delivered airflow. This calculator uses a stretch multiplier to show that penalty. A duct at full stretch gives the lowest calculated loss. A partly compressed duct gives a higher result.
Velocity and Flow Regime
The calculator also reports velocity and Reynolds number. Reynolds number describes the flow pattern. Most practical duct systems operate in turbulent flow. Turbulent flow needs a friction factor based on roughness and Reynolds number. The result is then combined with dynamic pressure and effective length.
Using the Result
Total pressure loss is shown in pascals and inches of water gauge. The friction rate is also shown per 100 feet. This value is useful when comparing duct sizes. A lower friction rate often means quieter operation and better airflow. The chart shows how loss changes as airflow rises or falls. Export the results when preparing reports, service notes, or design comparisons.
Frequently Asked Questions
1. What does flexible duct friction loss mean?
It is the pressure drop caused by airflow rubbing against the duct surface and passing through bends. Higher loss means the fan needs more pressure to deliver the same airflow.
2. Why does flexible duct have higher loss?
Flexible duct has a ribbed inner surface. It can also sag or compress. These conditions disturb airflow and increase resistance compared with smooth, straight metal duct.
3. What is duct stretch percentage?
It describes how fully the flexible duct is extended. A 100 percent value means fully stretched. Lower values represent compression and usually increase pressure loss.
4. What is a bend loss coefficient?
It is a K value for local pressure loss at a bend. Larger or sharper bends usually have higher K values and greater airflow resistance.
5. Why is Reynolds number shown?
Reynolds number helps identify the flow regime. It also supports friction factor selection. Most duct calculations fall in the turbulent range.
6. Can this replace a full duct design?
No. It is a planning and checking tool. Final designs should also consider codes, manufacturer data, balancing needs, noise limits, and site conditions.
7. Why include a safety factor?
A safety factor adds margin for installation differences, fittings, rough field conditions, and uncertain input data. It gives a more conservative design result.
8. What unit is inches of water gauge?
Inches of water gauge is a common HVAC pressure unit. It describes pressure using the height of a water column moved by that pressure.