Calculator Inputs
Use a single-column page flow with a responsive input grid.
Plotly Graph
The chart compares friction rate and velocity across round-equivalent diameters near the solved size.
Example Data Table
| Run | Airflow | Target Friction | Suggested Round | Suggested Rectangular | Comment |
|---|---|---|---|---|---|
| Main A | 4,000 CFM | 0.10 in. w.g./100 ft | 22 in | 28 × 14 in | Balanced main trunk example. |
| Main B | 2,500 CFM | 0.08 in. w.g./100 ft | 19 in | 24 × 12 in | Lower friction for quiet supply service. |
| Branch C | 1,200 CFM | 0.10 in. w.g./100 ft | 14 in | 18 × 10 in | Typical branch comparison row. |
| Return D | 1,800 CFM | 0.10 in. w.g./100 ft | 17 in | 22 × 10 in | Return run with moderate velocity. |
| Exhaust E | 900 CFM | 0.12 in. w.g./100 ft | 12 in | 16 × 8 in | Compact exhaust route example. |
Formula Used
This page sizes a duct by matching airflow and target friction loss. The round solution is solved iteratively, then rectangular sizes are derived from the round-equivalent result.
Where ΔP/L is pressure drop per unit length, f is the Darcy friction factor, ρ is air density, V is air velocity, and D is the duct diameter.
The Swamee–Jain relation estimates the Darcy friction factor for turbulent flow. For laminar flow, the calculator switches to f = 64/Re.
For rectangular ducts, the calculator uses hydraulic diameter Dₕ with width a and height b to estimate friction performance.
The rectangular option also uses the Huebscher equivalent-diameter relation so the rectangular size tracks the same flow resistance as the solved round duct.
How to Use This Calculator
- Choose imperial or metric units for the project.
- Select round or rectangular duct output.
- Enter airflow, target friction rate, and equivalent length.
- For rectangular ducts, enter the preferred width-to-height aspect ratio.
- Adjust roughness, density, viscosity, rounding increment, and advisory velocity when needed.
- Press Calculate Duct Size to show the result block above the form.
- Review actual friction, pressure drop, Reynolds number, and the Plotly graph.
- Download a CSV or PDF summary for handoff or documentation.
FAQs
1. What does duct sizing by friction mean?
It means choosing the duct size that delivers a target airflow while staying close to a selected pressure-loss rate per unit length.
2. Why is friction rate important?
Friction rate affects fan energy, total static pressure, noise, and available space. Lower friction usually means larger ducts and lower velocity.
3. Does this calculator support rectangular ducts?
Yes. It first solves the round-equivalent diameter, then converts that result into a rectangular size using your chosen aspect ratio.
4. What length should I enter?
Enter the equivalent length for the duct run you are evaluating. Include fitting effects in your equivalent-length estimate when possible.
5. Why does the rounded size differ from the exact size?
Fabricated ducts use practical commercial increments. The calculator rounds upward so the selected size stays at or below the target resistance more safely.
6. What is the Reynolds number shown in results?
It indicates the flow regime. Higher values usually mean turbulent flow, where roughness and friction-factor modeling become especially important.
7. Can I use this for return or exhaust ducts?
Yes. Use the friction target and advisory velocity appropriate for your system type, balancing noise, energy, and available space.
8. Is the graph useful for design review?
Yes. It helps you see how sensitive friction and velocity are to diameter changes near the selected solution.