Fan Affinity Laws Calculator

For the same fan and system, flow scales with speed, pressure with speed squared and power with speed cubed.

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Formula

CFM₂ = CFM₁ × (N₂/N₁)
SP₂ = SP₁ × (N₂/N₁)²
BHP₂ = BHP₁ × (N₂/N₁)³

Example calculation

1,000 CFM at 1,000 rpm → 1,100 rpm gives 1,100 CFM and 1.33× power.

Practical HVAC use

Pulley changes and blower speed tap decisions.

Common mistakes

Applying the laws when the duct system also changes.

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Educational tool. Results are estimates from the stated formulas; always confirm with manufacturer data and the code adopted in your jurisdiction. All calculators