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
How to use the duct calculator
The calculator works in three modes, and you pick the one where the value you are missing is the answer. In every mode the field being calculated is greyed out, and the remaining inputs stay adjustable with the sliders or by typing an exact number.
- Diameter — use this when you know how much air has to move and the air velocity you want. Typical for new installations: you have the air volume from the process or the room, you choose a velocity, and you get the duct size you need.
- Air velocity — use this when the ducting already exists. Enter the installed diameter and the air volume, and you see how fast the air is actually travelling in that duct.
- Air volume — use this to find the capacity of a given duct. Enter the diameter and the air velocity, and you get the air volume in m³/h. Useful when you want to know what a duct can carry before you add another extraction point to it.
The duct length input does not change the diameter, velocity or air volume. It only converts the pressure loss per metre into a total pressure loss for that length of straight duct.
What pressure loss means and why it matters
Pressure loss is the pressure the fan has to overcome to push air through the ducting. The calculator shows it two ways. Pa/m is the loss per metre of straight duct — a property of the duct size and the air velocity, independent of how long the run is. Total Pa is that figure multiplied by the duct length you entered, so it is the loss for the specific run you are looking at.
Three things drive pressure loss up: higher air velocity, smaller diameter, and greater length. Velocity is the strongest of the three, because the loss grows with roughly the square of the velocity. The figures here cover straight duct only — bends, transitions, dampers, hoods and filters add their own losses, and you take those from the component data sheets and add them to the total.
The friction values behind the calculation assume normal galvanised ducting in clean condition. A duct coated inside with dust, grease or chips will have a higher real loss than the calculated figure.
Spiro or transport duct — which should I choose
The duct type selector does not change the calculation. It changes which standard sizes the "We recommend — Ø mm" buttons offer you, so the size you land on is one you can actually order. The maths is identical whichever type you pick.
- All — every size in the calculator, from Ø32 to Ø1250 mm. Use it when you want the full range or are working with a size not in the two lists below.
- Spiro — the standard spiral-wound sizes: 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800 and 1000 mm. This is the usual choice for general ventilation ducting.
- Transport — the smaller sizes: 32, 38, 50, 63, 76, 80 and 100 mm. These are the diameters used for extraction arms, hoses and point extraction, where the air volume per point is small.
Recommended air velocities
Typical design ranges by application. Treat them as a starting point, then check the resulting pressure loss in the calculator.
| Application | Air velocity | Notes |
|---|---|---|
| Main ducts (supply and extract) | 8–12 m/s | Carries the full air volume; higher velocity means smaller ducts but more pressure loss. |
| Branch ducts | 5–8 m/s | Usually designed lower than the main duct to keep noise down near occupied areas. |
| Local extraction (arms, hoods) | 10–15 m/s | Needs enough velocity at the capture point to draw in fumes, dust or mist. |
| Chip and dust extraction | 18–25 m/s | Must stay above the transport velocity for the material, or the duct settles and clogs. |
Frequently asked questions
How do I calculate the air volume in a ventilation system?
Air volume follows from the duct cross-section and the air velocity in it: multiply the cross-sectional area in m² by the velocity in m/s, then by 3600 to get m³/h. In the calculator you do not have to do this by hand — pick the Air volume mode, set the diameter and the air velocity, and the result appears directly. For a whole system you normally add up the required air volume for each extraction point or room and size the main duct for the total.
How do I size a ventilation system?
Start from the air volume each point needs. Then choose the air velocity you want to design for, and let the calculator give you the required diameter. Round up to the nearest standard duct size, check the resulting air velocity and pressure loss for that real size, and add the duct length to see the total pressure loss in Pa. Repeat branch by branch and add the losses along the longest, most resistant path — that path decides the fan you need.
What is the relationship between diameter, air velocity and air volume?
The three are locked together: air volume equals cross-sectional area times air velocity. Because the area grows with the square of the diameter, a small change in diameter has a large effect. Keep the air volume fixed and make the duct smaller, and the air velocity rises steeply. Keep the velocity fixed and make the duct larger, and it carries much more air. That is why the calculator lets you solve for whichever of the three is unknown.
Why does pressure loss increase when I choose a smaller duct?
Two things happen at once. The same air volume through a smaller cross-section means a higher air velocity, and pressure loss grows roughly with the square of the velocity. At the same time, the loss per metre grows as the diameter shrinks, because the air is in contact with proportionally more duct wall. One step down in duct size can therefore raise the pressure loss substantially, not marginally.
What air velocity should I design for?
It is a trade-off. Low velocity gives low pressure loss, low noise and a smaller fan, but larger and more expensive ducts. High velocity saves space and material but costs fan power and can generate noise. Extraction of chips, dust or heavy particles has an extra constraint: the velocity must stay high enough to keep the material moving. Use the table above as a starting point and check the pressure loss the calculator returns.
How much air does an extraction fan need to move?
The fan has to deliver the total air volume of all points running at the same time, against the total pressure loss of the system. Use the calculator to get the pressure loss for each duct run, add the losses along the critical path, and add the losses of filters, hoods, bends and other components from their own data sheets. The result is a duty point — air volume in m³/h at a given pressure in Pa — which is what you select the fan against.