Ø124.78 mm
A0.012 m²
Pressure loss
2.01 Pa/m
Total pressure loss — 1.0 m
2.01 Pa
We recommend — Ø mm
Diameter
mm124.78
Pick formula
Duct type
Air velocity
m/s5.0
Air volume
m³/h220
Duct length
m1.0
Duct Sizing Calculator
Duct sizing is the step where a ventilation layout turns into an order list. You know how much air a room, machine or extraction point has to move, and you need to know which duct diameter carries that air without wasting fan power or filling the ceiling void. This page starts the calculator in Diameter mode, so the diameter is the answer and air volume and air velocity are the inputs you set.
The workflow is short. Enter the air volume in m³/h, set the air velocity you want to design for, and read the required diameter straight off the result panel. Because a duct in that exact theoretical size does not exist, the calculator also highlights the nearest standard size from the size list, and the duct type selector switches between the full range, the Spiro sizes and the smaller transport sizes so the suggestion is something you can actually buy.
Once you have a standard size, sanity-check it. Switch to Air velocity mode with that real diameter and the same air volume to see the velocity you actually end up with — rounding up one size lowers the velocity, rounding down raises it. Then look at the pressure loss figures: Pa per metre for the duct itself, and total Pa once you enter the length of the run. Doing this branch by branch gives you the numbers you need to select a fan later.
Size selection is always a compromise between duct cost, space, noise and fan energy. A generous duct costs more in sheet metal but less in electricity for the lifetime of the system; a tight duct is cheap to install and expensive to run. The calculator does not make that decision for you, but it puts both consequences on screen at the same time so the trade-off is visible while you are still choosing.
Frequently asked questions
How do I size a ventilation duct?
Start with the air volume the duct must carry in m³/h, choose the air velocity you want to design for, and read the required diameter. Then round to a standard size and re-check the resulting velocity and pressure loss for that real size.
Should I round a duct size up or down?
Rounding up gives a lower air velocity and lower pressure loss, at higher material cost. Rounding down does the opposite. For extraction of dust or chips, check that rounding up does not drop the velocity below the transport velocity the material needs.
Does duct type change the calculated diameter?
No. The duct type selector only filters which standard sizes are offered as a recommendation. The calculated diameter, velocity, air volume and pressure loss are identical for every duct type.
Can I size a whole system with this page?
You can size each individual run with it. For the complete system you size branch by branch, then add the pressure losses along the longest and most resistant path, including components such as filters, hoods and bends from their own data sheets.
What if the required diameter falls between two standard sizes?
That is the normal case. The calculator marks the nearest standard size for you; compare both neighbours by entering each one in Air velocity mode and comparing the resulting velocity and pressure loss.
Why does my existing duct not match the calculated size?
Existing installations are often sized for a different air volume than they now carry, or were sized to fit the space available. Enter the installed diameter in Air velocity mode to see what the current velocity really is.