A duty point is three numbers: how much air, how hard the fan has to push it, and the air density they apply at. Work down the page; each step feeds the next.
The density of the air at the fan inlet. Every later number depends on it, which is why it is the first thing asked and the one figure never assumed.
Pick the closest description and answer in things you can measure on site. This step works out the target volumetric flow rate in m³/hr. You do not need to know it already.
Walk the air's path from one end to the other. This step adds up everything it has to fight through, into the static pressure in mmWG.
A little headroom on top of the calculated figures. Always shown separately, never quietly folded in.
This calculator runs entirely in your browser. Nothing you type is sent anywhere, and no figure is stored. Duct friction is Darcy–Weisbach with the Colebrook–White friction factor at 0.09 mm absolute roughness for steel, and fitting losses use the ζ convention (ASHRAE writes C, ACGIH writes F; they are the same quantity). A free ductulator assuming a different roughness will give a slightly different figure, and that is why. It gives an indicative duty point and fan family; it is not a quotation and not a performance guarantee. Send us the printed sheet and we will confirm the selection against your actual system.
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