Fan laws calculator
Enter a known duty point — flow, pressure, absorbed power, air density, impeller diameter and speed — then a new speed, diameter and/or air density, and get the scaled flow, pressure and power at the new operating point using the standard fan affinity laws.
How to use this calculator
- Enter the known duty point: flow, pressure (static or total), power, density, impeller diameter and speed.
- Enter the new speed, diameter and/or density you want to scale to.
- Read the scaled flow, pressure and power at the new condition.
The fan affinity laws
For a geometrically similar fan — same impeller family, just a different speed, diameter or air density — the operating point scales predictably. These are the same three relationships used to size a fan family from one tested performance curve, or to predict how a duty point shifts when you change a pulley ratio, swap in a bigger impeller, or move an installation to altitude.
Q₂ = Q₁ · (N₂/N₁)· (D₂/D₁)³
P₂ = P₁ · (N₂/N₁)² · (D₂/D₁)² · (ρ₂/ρ₁)
W₂ = W₁ · (N₂/N₁)³ · (D₂/D₁)⁵ · (ρ₂/ρ₁)
Q is volume flow, P is pressure — static or total, the same law applies to either, as long as you stay consistent between the known and new point — W is absorbed power, N is rotor speed and ρ is air density.
Speed-only changes (same fan, same diameter, same air) are the most common case — set D₂ = D₁ and ρ₂ = ρ₁, and only N₂ changes. The "reset new conditions" button below the form does exactly that as a starting point.
What these laws don't capture
The affinity laws assume the new operating point sits at the same relative position on the fan's performance curve — same efficiency, same flow coefficient. They hold well for small-to-moderate changes in speed and are the standard way to rescale a whole size range from one tested curve, but they don't account for Reynolds number effects at very different scales, or for a system curve that doesn't scale the same way as the fan curve.