Fan Affinity Laws

What a fan's airflow, static pressure, and power do when you change its speed.

Run the calculator

Example

You enter

Baseline RPM 1750
Baseline CFM 5000
Baseline SP (in wc) 1
Baseline kW 5
Target RPM
Target RPM 1500

You get

Ratio 0.857
CFM 4285.7
SP 0.735 in wc
kW 3.149

Checked against published worked examples from: ASHRAE.

Details, formula, and sources

ScopeQ ~ N, P ~ N^2, kW ~ N^3 across RPM, CFM, SP, and kW changes.

Formula or tableQ2/Q1 = (N2/N1); H2/H1 = (N2/N1)²; P2/P1 = (N2/N1)³. Laws apply for geometrically similar fans / pumps at the same point of operation.

Source and editionHydraulic Institute / AMCA by name; classical pump and fan theory.

Free accessPrinciples free in published engineering texts.

What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

API field namesbaseline_RPM, baseline_CFM, baseline_SP_in_wc, baseline_kW, target_kind, target_RPM, ratio, CFM, SP_in_wc, kW

Operating point constant (no system-curve change)affinity-law assumption