Duct Velocity Calculator
Enter the airflow and the duct size and get the actual velocity in metres per second and feet per minute, checked against the recommended band for residential, low-noise commercial, general commercial, industrial or branch duty. Works for round, rectangular and flat oval sections, and reports velocity pressure alongside.
Air velocity in 18.0 in ø
5.75 m/s
1,132 fpm · velocity pressure 18.5 Pa
| Cross-sectional area | 0.1642 m² · 1.767 ft² |
| Circular equivalent diameter | 18.0 in |
| Velocity in the equivalent round duct | 5.75 m/s · 1,132 fpm |
| Hydraulic diameter / perimeter | 18.0 in / 56.5 in |
| Aspect ratio | n/a — round |
| Air density used | 1.1198 kg/m³ |
Velocity sits inside the 5–8 m/s band for commercial — general work.
Velocity is the number occupants actually notice
Nobody in a finished building ever complains about a friction rate. They complain about a hiss from a diffuser, a rumble in a bulkhead, or a meeting room where the system has to be switched off to hold a call. Every one of those traces back to velocity — and specifically to velocity at a fitting, where turbulence turns air speed into noise far more efficiently than straight duct ever does.
That is why velocity deserves a check independent of the sizing calculation. A duct sized correctly on equal friction can still be too fast at a take-off close to a quiet room, and a duct that reads fine on a schedule can be well outside its band once the real flow is measured on commissioning. Enter the flow you actually have and the size that is actually installed, and the band verdict tells you what the occupant will experience.
How to use the duct velocity calculator
- Enter the airflow for this section. The air actually carried by the piece of duct in front of you, not the fan total. On a branch, that is the branch flow.
- Choose the shape and enter the internal size. Internal clear dimensions. A 600 mm duct with 25 mm internal lining is a 550 mm air path, and the velocity follows the air path.
- Pick the application. This sets the band the result is judged against. Residential and hospital work tolerate far less velocity than an industrial main.
- Check the temperature and elevation if they are unusual. They do not change the velocity — that is pure geometry — but they set the air density and therefore the velocity pressure, which is what a pitot traverse reads.
- Read the verdict, not just the number. The band check tells you whether the velocity is quiet, acceptable or going to be heard in the room. That is usually the decision you are actually trying to make.
Formulas and a worked example
- Velocity: V = Q ÷ A
- Round area: A = πD² ÷ 4
- Rectangular area: A = a × b
- Flat oval area: A = πa²/4 + a(W − a)
- Velocity pressure: pv = ρV² ÷ 2
- Density: ρ = p ÷ (0.287 T), with p from the standard atmosphere at your elevation
Worked example
0.5 m³/s through a 300 mm round duct. Area = π × 0.3² ÷ 4 = 0.070686 m², so V = 0.5 ÷ 0.070686 = 7.07 m/s, which is 1392 fpm. At sea level and 20 °C the density is 1.2043 kg/m³, so the velocity pressure is 1.2043 × 7.07² ÷ 2 = 30.1 Pa. Against the 5–8 m/s band for general commercial work that velocity is acceptable; against the 4–6 m/s low-noise band it is not, and near a quiet space the duct would want to be one size larger.
Recommended velocity bands
| Application | m/s | fpm |
|---|---|---|
| Residential | 3–5 | 591–984 |
| Commercial — low noise | 4–6 | 787–1181 |
| Commercial — general | 5–8 | 984–1575 |
| Industrial | 8–12 | 1575–2362 |
| Branch / runout | 2–4 | 394–787 |
Main duct figures. The full breakdown by position in the system — riser, branch, runout and outlet — is on the duct velocity chart.
Frequently asked questions
How do you calculate air velocity in a duct?
Divide the volume flow rate by the cross-sectional area of the duct: V = Q ÷ A. In metric, airflow in cubic metres per second divided by area in square metres gives metres per second. In imperial, CFM divided by area in square feet gives feet per minute directly, which is why fpm remains the working unit on North American drawings. The only trap is the area — use the internal clear dimensions, and subtract any internal lining.
What is a good air velocity for a duct?
It depends entirely on where the duct is. A main serving general commercial space usually runs 5 to 8 m/s (1000 to 1600 fpm). Where noise matters — offices, hospitals, studios — 4 to 6 m/s is the working band. Branches and final runouts drop to 2 to 4 m/s so the diffuser, not the duct, sets the sound level. Industrial mains run 8 to 12 m/s and sometimes higher where particulate transport demands it.
Why is my rectangular duct velocity different from the equivalent round velocity?
Because the circular equivalent diameter is defined on an equal-friction, equal-airflow basis rather than equal area. The equivalent round duct is smaller in area, so the same air moves through it faster. The real section velocity — the one this calculator leads with — is the figure for noise and terminal selection; the equivalent velocity belongs to friction calculations.
Does duct velocity change with temperature or altitude?
Not if the volume flow is fixed, because velocity is simply volume flow divided by area. What does change is the density, and therefore the velocity pressure, the mass of air delivered and the pressure loss. This matters practically: a pitot tube measures velocity pressure and converts it using an assumed density, so a traverse taken at altitude with sea-level density assumed will read the velocity wrong.
What is velocity pressure and why is it shown?
Velocity pressure is the kinetic energy of the moving air, ρV²/2, and it is what a pitot tube actually measures. It also drives fitting losses, which are expressed as a coefficient multiplied by the velocity pressure. Doubling the velocity quadruples it — which is the real reason high-velocity systems get expensive at every elbow, not just along the straight duct.
How do I reduce velocity that is too high?
Increase the duct area. Going up one standard size is the direct fix; on a rectangular duct you can add width if the void depth is fixed, but watch the aspect ratio, because a flatter duct with the same area buys you the velocity reduction at the cost of more friction, metal and insulation. If the run is short and the noise is local, an attenuator or a lined section can be cheaper than resizing the whole run.
Last updated: 26 July 2026