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
Pressure Drop Calculator
Pressure drop is what the fan has to work against. Every metre of ducting takes a share of the available pressure, and when the shares add up to more than the fan can deliver, the system simply does not move the air it was designed for. This page gives you the pressure drop for a duct run directly: the loss per metre in Pa/m and the total loss in Pa once you enter the length.
The calculator shows both figures at all times, in every mode, so you can approach the run from whichever side you know. If the diameter is fixed by the building, set it and vary the air volume. If the air volume is fixed by the process, set that and try different diameters. In each case the panel updates the Pa/m and total Pa figures, so the pressure consequence of a decision is visible while you are making it rather than after installation.
Pa/m is a property of the duct and the flow through it, independent of length — it tells you how expensive a particular size is per metre. Total Pa is that value multiplied by the length you entered, and it is the figure you carry forward when you add up a system. Work along the longest and most resistant path from the fan to the furthest point, add the totals for each straight section, then add the losses for filters, hoods, bends, dampers and silencers from their manufacturers' data. The sum is the static pressure the fan must produce.
One practical note on accuracy: these figures cover clean, straight ducting. Real systems drift upwards over time as filters load and duct walls collect dust or grease, and every fitting adds a local loss that straight-duct maths does not include. Treat the calculated total as the baseline of a system in good condition and allow for the components and the condition of the installation on top of it.
Frequently asked questions
What does Pa/m mean?
Pascals of pressure loss per metre of straight duct. It depends on the duct diameter and the air velocity, not on how long the run is. Multiply it by the length to get the loss for a specific section.
Why does pressure loss increase when I choose a smaller duct?
Two effects add up. The same air volume through a smaller cross-section travels faster, and loss grows with roughly the square of the velocity. In addition, the loss per metre rises as the diameter falls, because the air meets proportionally more duct wall.
Does this include bends, filters and hoods?
No. The figures are for straight ducting only. Component losses come from the manufacturers' data sheets and are added to the total.
How do I get the total pressure for the whole system?
Follow the most resistant path from the fan to the furthest point, add the total Pa for each straight section along it, then add the losses of the components in that path. That sum is the pressure the fan must overcome.
How much does a longer duct run cost me?
In straight duct the relationship is proportional: double the length and you double the loss for that section at the same air volume and diameter. Enter different lengths in the duct length field to see it directly.
Why is the measured pressure drop higher than the calculated one?
Common reasons are fittings and components that straight-duct calculation excludes, a duct that is dirtier than the clean condition assumed, or an actual air volume higher than the one entered. Check the real air volume and velocity first.