💨 Fan Laws Calculator
Airflow, pressure & power at new speed or wheel diameter
Results at New Speed
Speed Ratio
—
N₂/N₁
New Airflow Q₂
—
CFM
New Pressure SP₂
—
in w.g.
New Power BHP₂
—
HP
Fan Laws (same as pump affinity laws)
Q₂ = Q₁ × (N₂/N₁) (flow ∝ speed)
SP₂ = SP₁ × (N₂/N₁)² (pressure ∝ speed²)
BHP₂ = BHP₁ × (N₂/N₁)³ (power ∝ speed³)
📝 Worked Example
Fan at 1200 RPM: 10,000 CFM, 2.5 in w.g., 8 BHP. Reduce to 900 RPM for part-load.
Ratio = 900/1200 = 0.75
Q₂ = 10,000 × 0.75 = 7,500 CFM | SP₂ = 2.5 × 0.75² = 1.41 in w.g.
BHP₂ = 8 × 0.75³ = 3.38 BHP → 57.8% power saving!
7,500 CFM | 1.41 in w.g. | 3.38 BHP