Predict a fan's flow, pressure and absorbed power at a new speed using the fan affinity laws: flow ∝ speed, pressure ∝ speed², power ∝ speed³.
At constant impeller diameter and air density. Verify against the manufacturer fan curve.
For design guidance only. Always verify against the manufacturer fan curve.
About this fan affinity laws calculator
This free fan affinity laws calculator predicts how a fan's duty changes when its speed changes, for example when a variable-speed drive (VSD) trims an air-handling unit's flow. From a known reference duty (volume flow Q₁, fan pressure P₁ and absorbed power W₁ at a measured speed N₁), it works out the new flow, pressure and power at a target speed N₂. It is built for building-services and ventilation engineers sizing fans, assessing energy savings, or checking a VSD setpoint. Everything runs in your browser. Nothing is uploaded.
How the fan affinity laws are calculated
The affinity (fan) laws relate fan duty to the speed ratio r = N₂ ÷ N₁ at constant impeller diameter and air density. Volume flow scales with speed, pressure with the square of speed and power with the cube: Q₂ = Q₁·r, P₂ = P₁·r² and W₂ = W₁·r³. Power follows because it is the product of flow and pressure (r¹·r² = r³), which is why a modest speed cut yields a large power saving, a 20% speed reduction (r = 0.8) gives 80% flow, 64% pressure and about 51% power. The relationships are stated in AMCA Publication 201, “Fans and Systems”, and hold to roughly 2–5% within about 25–30% of the reference speed; beyond that the calculator flags an advisory to verify against the manufacturer fan curve.
Frequently asked questions
Why does power fall so much faster than flow?
Absorbed power is proportional to the cube of speed, so halving the speed cuts the power demand to about an eighth (0.5³ = 0.125). This cubic relationship is the reason variable-speed fans deliver such large energy savings at part load.
When do the fan laws stop being accurate?
They assume constant impeller diameter and constant air density, and they ignore changes in fan and motor efficiency. They stay within a few per cent for speed changes up to roughly 25–30%; for larger changes, or near the fan's stall region, read the duty straight off the manufacturer's fan curve.
Can I use these laws for a different fan diameter?
Not directly. This calculator models a speed change on one fan only. The full affinity laws include separate diameter-ratio and density-ratio terms; for a change of impeller size or air density you need those additional factors.