Fan Law Calculator

Apply the three fan affinity laws to calculate new fan performance at different speeds.

Known Condition (Point 1)

New Condition (Point 2)

Speed Ratio
--
RPM2 / RPM1
New Flow (CFM)
--
Q2 = Q1 x (N2/N1)
New Pressure (in. w.g.)
--
P2 = P1 x (N2/N1)^2
New Power (BHP)
--
HP2 = HP1 x (N2/N1)^3
New Speed (RPM)
--
Power Savings
--

The Three Fan Laws

Law 1: Q2/Q1 = N2/N1 (flow proportional to speed)

Law 2: P2/P1 = (N2/N1)^2 (pressure proportional to speed squared)

Law 3: HP2/HP1 = (N2/N1)^3 (power proportional to speed cubed)

These laws apply when the fan diameter and air density remain constant. The cubic relationship of power means a small speed reduction yields large energy savings.

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