Tools / Fan Laws

Fan Laws

Enter a known operating point. A speed change scales Q ∝ N, P ∝ N², HP ∝ N³. An impeller-diameter change at constant speed scales Q ∝ D³, P ∝ D², HP ∝ D⁵. Optional density correction is from elevation and temperature against 70 °F at sea level.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms

Known condition

Flow (cfm)Speed (rpm)Static pressure (in. wg)Power (hp)Impeller diameter (in)

Change

New speed (rpm)

Density correction (elevation and temperature)

Condition 2 (predicted)

Flow2000 cfm

Speed3500 rpm

Pressure8.000 in. wg

Power8.000 hp

Ratios

Flow2.000×

Pressure4.000×

Power8.000×

Speed change: Q ∝ N, P ∝ N², HP ∝ N³. Diameter change at constant speed: Q ∝ D³, P ∝ D², HP ∝ D⁵. Density correction uses 70 °F / sea level as standard. Affinity assumes the same system curve (resistance ∝ Q²) — it is not a fan-curve / system intersection. Affinity assumes the same system curve (resistance ∝ Q²) — it is not a fan-curve / system intersection.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms