Ø125.00 mm
A0.012 m²
Pressure loss
1.99 Pa/m
Total pressure loss — 1.0 m
1.99 Pa
We recommend — Ø mm
Air velocity
m/s4.98
Pick formula
Duct type
Air volume
m³/h220
Diameter
mm125
Duct length
m1.0
Air Velocity Calculator
Air velocity is the figure that decides whether a duct is comfortable, quiet and efficient — or noisy and expensive to run. This page opens the calculator in Air velocity mode: you enter the duct diameter and the air volume passing through it, and the velocity in metres per second is the result. It is the fastest way to judge an installation you did not design yourself.
The most common use is diagnosis. A duct is already in the ceiling, the air volume is known from the design documents or from a measurement, and the question is whether the air is moving too fast. High velocity shows up as whistling at fittings, rumble in the duct wall and a fan that draws more power than expected. Low velocity is quieter, but in extraction systems it can mean dust, chips or condensate settle in the duct instead of being carried along.
Reading the result together with the pressure loss panel is where the value is. Velocity and pressure loss are tied together: as you increase the air volume through the same duct, the velocity climbs and the pressure loss per metre climbs faster still. Entering the duct length turns that into a total figure in Pa for the run, which tells you how much of the fan's available pressure this single duct is consuming.
If the velocity comes out uncomfortably high, there are only three levers: move less air, split the flow across more ducts, or use a larger duct. Switching to Diameter mode with your target velocity shows what size would be needed instead, so you can quantify the fix before proposing it.
Frequently asked questions
How do I calculate the air velocity in a duct?
Divide the air volume by the duct cross-sectional area, converting hours to seconds. Enter the diameter and the air volume in m³/h above and the calculator returns the velocity in m/s.
What air velocity should I design for?
It depends on the application: main ducts are normally designed faster than branches near occupied rooms, while extraction of chips and dust needs a minimum velocity to keep the material moving. Balance the noise and pressure loss of a fast duct against the cost and space of a slow one.
Why is my duct noisy?
Excessive air velocity is a common cause, especially at bends, dampers and grilles where turbulence is generated. Calculate the actual velocity first; if it is high, a larger duct or a lower air volume is the structural fix.
What is the relationship between diameter, air velocity and air volume?
Air volume equals cross-sectional area times velocity, and the area grows with the square of the diameter. So with the air volume fixed, a modest reduction in diameter produces a steep rise in velocity.
Can the velocity be too low?
In general ventilation a low velocity is mostly a cost issue. In extraction systems it is a functional problem: below the transport velocity for the material being carried, dust or chips settle out and the duct gradually blocks.
Is this the velocity at the centre of the duct or the average?
The calculation gives the mean velocity across the full cross-section. A handheld measurement taken in the middle of a duct will usually read higher than that mean.