Fan Affinity Laws Calculator

Scale industrial fan airflow, static pressure, and motor brake horsepower across speed (RPM), wheel trim diameter, and gas density changes per AMCA standards.

Base Operating Condition 1

Base Speed N1 (RPM):

Base Airflow Q1 (CFM):

Base Static Pressure SP1 (in. w.g.):

Base Brake Horsepower BHP1 (HP):

New Target Operating Condition 2

New Speed N2 (RPM):

Impeller Diameter Ratio (D2 / D1):

Gas Density Ratio (ρ2 / ρ1):

Affinity Law Scaling Results

New Required Brake Horsepower (BHP2)

-- BHP

--x power increase

New Airflow (Q2)

-- CFM

--% change

New Pressure (SP2)

-- in. w.g.

--% change

Speed Scaling Ratio (N2 / N1): --

Power Cubed Multiplier (rN³): --

Motor Overload Warning: Normal

The cubic power law means even a modest 20% speed increase demands a 73% increase in electrical motor horsepower.

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Frequently Asked Questions

What are the three fundamental Fan Affinity Laws?

Law 1 states airflow CFM varies directly with fan speed RPM (Q ∝ N). Law 2 states static pressure varies with the square of speed (SP ∝ N²). Law 3 states brake horsepower varies with the cube of speed (BHP ∝ N³). Scaling impeller diameter D scales CFM with D³, pressure with D², and BHP with D⁵.

Why does a 20% fan speed increase require a 73% larger motor?

Because fan power follows the cubic law (N³), multiplying speed by 1.20 scales power demand by 1.20³ = 1.728. Thus, a modest 20% speed bump demands 72.8% more brake horsepower.

Can fan affinity laws be used if duct dampers are modulated?

No. Fan affinity laws strictly apply along a constant system resistance curve where the physical duct layout remains unchanged. If dampers are throttled or filter bags foul, the system resistance curve shifts and must be analyzed using the intersection with the fan curve.