Calculate fan or duct air velocity from airflow in CFM and the fan or duct size, or find the airflow or diameter needed, in ft/min, m/s or other units.
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Fan Velocity Formula
- V — air velocity (ft/min, convert to ft/s by dividing by 60)
- Q — volumetric airflow (CFM, ft³/min)
- A — cross-sectional area of the fan or duct (ft²)
- D — fan or duct diameter (ft)
To solve for diameter when velocity is the target, rearrange to D = 2 × √(Q / (V × π)). The diameter inverse uses a round flow area. V = Q / A gives the area-average normal velocity and does not require a uniform profile or standard air density. Use actual volumetric flow and the corresponding net flow area; standard-volume flow may require conversion to actual conditions.
Velocity Conversion Examples
The following table converts illustrative velocity ranges only; it does not recommend velocities for a particular system. Duct design, noise, contaminant transport and ventilation adequacy require appropriate project-specific criteria.
| Velocity (ft/min) | Velocity (ft/s) |
|---|---|
| 300–500 | 5–8.333333 |
| 500–700 | 8.333333–11.666667 |
| 700–1000 | 11.666667–16.666667 |
| 1200–1800 | 20–30 |
| 2100–3000 | 35–50 |
| Diameter | Area (ft²) | CFM at 10 ft/s |
|---|---|---|
| 6 in | 0.196 | 118 |
| 8 in | 0.349 | 209 |
| 10 in | 0.545 | 327 |
| 12 in | 0.785 | 471 |
| 16 in | 1.396 | 838 |
| 24 in | 3.142 | 1885 |
Example
A 12-inch duct moves 1200 CFM. What is the air velocity?
- Convert diameter to feet: 12 in ÷ 12 = 1 ft
- Area = π × (0.5)² = 0.785 ft²
- Velocity = 1200 ÷ (π/4) ≈1527.887 ft/min
- Divide by 60: 25.5 ft/s (7.76 m/s)
This is an average through the specified area. It does not establish an acceptable noise level, fan operating point, duct size or room ventilation rate.
FAQ
Why does my measured velocity differ from this calculator? A point reading can differ from an area average because flow profiles vary with obstructions, swirl and measurement location. Use an appropriate traverse and measurement method for the installation.
Does this work for rectangular ducts? Yes. Select Rectangular duct and enter width and height. Known net free area is also supported. The airflow mode uses the same geometries; the diameter mode returns a round flow-area diameter.
What velocity is too high? There is no universal limit set by this calculator. Apply the relevant system design, acoustic, pressure-loss and contaminant-transport criteria; average velocity alone does not establish suitability.
