Vortia 3DDuct air velocity calculator
Enter the airflow and the duct section to see the air velocity, whether it is within the recommended range for that type of run and which standard diameter you need for the velocity you want.
How it is calculated
v = Q / A · A = π·D² / 4 · pd = ρ·v² / 2Q in m³/s (m³/h ÷ 3,600), A in m², D in m, ρ ≈ 1.2 kg/m³ at 20 °CIn a rectangular duct, A = a·b. The equivalent Ø is the circular duct with the same friction loss at the same airflow; it is used to compare sections.
For the diameter, D = √(4·Q / (π·v)) is solved and the size from the standard series is taken: 80, 100, 125, 160, 200, 250, 280, 315, 355, 400, 450, 500, 560, 630 mm…
Recommended velocities
| Use | Main duct | Branches and final runs |
|---|---|---|
| Residential (dwellings) | 3–4 m/s | 2–3 m/s |
| Non-residential (offices, retail) | 5–8 m/s | 2.5–4 m/s |
| Industrial (supply) | 10–15 m/s | 5–8 m/s |
Indicative design practice values (ASHRAE Fundamentals). In dust or fume extraction, the minimum transport velocity takes precedence.
Example
1,000 m³/h through a Ø250 mm duct: A = 0.0491 m² and v = 0.278 / 0.0491 = 5.66 m/s, with 19.3 Pa of velocity pressure. For an office it is fine in the main and high for a branch. For exactly 5 m/s you would need 266 mm: the next standard size up is Ø280 (4.51 m/s).
Bear in mind
- The velocity is the average across the section; near bends and branches the profile is not uniform.
- High velocity = more pressure drop (it grows with v²) and more noise. Low velocity = more sheet metal and more ceiling void space.
- Velocity is not the whole story: to compare long runs, also look at the pressure drop per metre.