Duct reference
Equivalent round duct diameter table
Equal-friction approximation examples for comparing a rectangular duct with a round duct under the stated formula assumptions.
Lookup examples
| Rectangular duct (in) | Face area (in²) | Equivalent round (in) |
|---|---|---|
| 12 × 8 | 96 | 10.7 |
| 16 × 8 | 128 | 12.7 |
| 18 × 10 | 180 | 15.4 |
| 24 × 12 | 288 | 18.1 |
| 30 × 12 | 360 | 20.8 |
Formula and variables
W and H are rectangular duct dimensions in inches; Dₑ is equivalent round diameter in inches. This Huebscher equal-friction approximation compares friction behavior under its assumptions. It is not an equal-area conversion and does not include fittings, transitions, leakage, or terminals.
How to use it
- Use actual inside dimensions and calculate the equivalent round diameter.
- Check airflow, actual velocity, and the selected equal-friction or other design method.
- Evaluate fittings, transitions, aspect ratio, and route constraints separately.
- Check the complete resistance against fan performance before final selection.
Example interpretation
A 24 × 12 in rectangular duct has an approximate 18.1 in equivalent round diameter by this formula. That does not prove the two installed paths have the same total pressure loss: elbows, takeoffs, transitions, length, and construction still matter.
FAQ
Can I use this instead of a complete duct calculation?
No. Confirm final pressure loss, velocity, fittings, duct construction, and system requirements with the appropriate design method.