Ø125.00 mm
A0.012 m²
Pressure loss
2.00 Pa/m
Total pressure loss — 1.0 m
2.00 Pa
We recommend — Ø mm
Air volume
m³/h220.78
Pick formula
Duct type
Air velocity
m/s5.0
Diameter
mm125
Duct length
m1.0
Air Volume Calculator
Air volume is the number that describes what a ventilation system actually delivers: how many cubic metres of air pass a given point every hour. This page opens the calculator in Air volume mode, where diameter and air velocity are the inputs and the m³/h figure is the result. It answers the question that comes up whenever ducting already exists — what can this duct carry?
Typical situations where you want the air volume rather than the diameter: a workshop wants to add another extraction point to an existing branch and you need to know whether there is capacity left; a service visit measures the velocity in a duct and you want to convert that reading into a flow figure; or a quote has to state a capacity in m³/h for a duct size that has already been decided by the building.
Enter the installed diameter in millimetres and the air velocity in metres per second, and the result panel shows the air volume together with the duct cross-section in m². The same panel gives the pressure loss per metre and the total pressure loss for the duct length you enter, so you can see immediately what the flow costs in pressure. If the number is not what you hoped for, adjusting the velocity slider shows how much extra flow a faster — and noisier — duct would give you.
Remember that the air volume shown here is what the duct carries at the velocity you entered. It is a geometric relationship, not a promise that the fan can deliver it. Whether that flow is achievable depends on the total pressure loss of the system and the fan curve, which is why it is worth reading the Pa figures next to the m³/h result rather than in isolation.
Frequently asked questions
How do I calculate the air volume in a ventilation system?
Multiply the duct cross-sectional area in m² by the air velocity in m/s and then by 3600 to get m³/h. Enter diameter and velocity above and the calculator does this for you.
How much air does an extraction fan need to move?
It has to cover the total air volume of every point running simultaneously, delivered against the system's total pressure loss. Add the required volumes, then add the pressure losses along the critical path to get a duty point in m³/h at a given Pa.
What is the difference between m³/h and l/s?
They measure the same thing in different units. One litre per second equals 3.6 cubic metres per hour. This calculator works in m³/h throughout.
Does a bigger duct move more air by itself?
Not on its own — a larger duct increases the capacity at a given velocity, but the actual flow is set by the fan working against the system resistance. A larger duct helps mainly by lowering that resistance.
Why does the air volume change when I move the velocity slider?
Because air volume is area times velocity. With the diameter fixed, the flow is directly proportional to the velocity: double the velocity and you double the m³/h.
Can I use the result to check a measurement on site?
Yes. Enter the duct diameter and the velocity you measured, and compare the calculated m³/h with the design figure for that branch. Bear in mind that a single-point velocity reading is not the same as the average velocity across the duct.