Fan Law (Affinity) Calculator

Inputs

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Scale from a known speed change, or back-solve the speed needed for a target airflow.
Measured or rated airflow at the baseline speed.
cfm
Fan (not motor) RPM at the baseline point.
rpm
Static (or total) pressure at the baseline point on the same system.
in. w.g.
Fan shaft (brake) power at the baseline point.
hp
Speed after the sheave change or VFD adjustment.
rpm

Results

Default result
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New airflow(Q₂)
1,250cfm

Law 1: Q₂ = Q₁ × (N₂/N₁).

Also computed

New static pressure(P₂)0.7841in. w.g.

Law 2: P₂ = P₁ × (N₂/N₁)².

New brake power(W₂)Out of tol.1.954hp

Power rises with the cube of speed: 95% more brake power. Check the motor, drive and electrical circuit before making this change.

Law 3: W₂ = W₁ × (N₂/N₁)³.

Fan speed(N₂)1,000rpm

Speed ratio(N₂/N₁)1.25

Airflow change(ΔQ)25%

Static change(ΔP)56.3%

Method notes 4 notes
  • Affinity laws hold for the SAME fan on the SAME system (fixed system curve, constant air density). Changing dampers, filters or ductwork moves the system curve and breaks the simple scaling.
  • Law 1: Q ∝ N. Law 2: P ∝ N². Law 3: W ∝ N³ — a 10% airflow increase costs ~33% more power.
  • Density matters: for altitude or hot-gas service, correct pressure and power by the density ratio before applying the laws.
  • Belt-drive changes follow sheave ratio; confirm the new fan RPM and the motor stays within its service factor.

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