Calculate fan speed, airflow, pressure, and power changes using the three fan affinity laws. Compare operating points for HVAC and industrial fans.
The Fan Law Calculator applies the three affinity laws to predict how changes in fan speed or impeller diameter affect airflow, pressure, and power consumption. These laws are essential for HVAC design, industrial ventilation, and energy optimization, especially when you are comparing VFD settings, belt changes, or fan wheel swaps. It gives a quick estimate of whether a proposed speed change is even in the right operating range.
The fan laws state that airflow varies linearly with speed (Q ∝ N), pressure varies with the square (P ∝ N²), and power varies with the cube (HP ∝ N³). This cubic relationship means that a 20% speed reduction cuts power consumption by nearly 50% — making variable frequency drives (VFDs) one of the best energy investments in building systems.
Enter your current fan operating conditions and the new speed or diameter, and the calculator instantly shows the new airflow, pressure, power, and efficiency. The comparison table reveals how sensitive each parameter is to speed changes.
Use this calculator when you need a first-pass estimate of what a speed change will do to airflow, pressure, and power before touching a VFD or fan sheave. It is useful for energy-savings studies, troubleshooting, and comparing how much performance you give up for a lower operating cost in HVAC or process fans.
Fan Law 1: Q₂/Q₁ = N₂/N₁. Fan Law 2: P₂/P₁ = (N₂/N₁)². Fan Law 3: HP₂/HP₁ = (N₂/N₁)³. Where Q = airflow (CFM), P = pressure (in. w.g.), HP = power, N = speed (RPM). Same laws apply with diameter ratio D₂/D₁ when speed is constant.
Result: 8,000 CFM, 1.28 in.w.g., 2.56 HP
Speed ratio = 960/1200 = 0.8. New CFM = 10,000 × 0.8 = 8,000. New pressure = 2.0 × 0.8² = 1.28 in.w.g. New power = 5.0 × 0.8³ = 2.56 HP. Power drops 49% while airflow only drops 20%.
Fan Law 1 (Flow): Doubling fan speed doubles airflow. This linear relationship is intuitive — spin the impeller faster and it moves more air proportionally. Fan Law 2 (Pressure): Doubling speed quadruples pressure because centrifugal force increases with the square of velocity. Fan Law 3 (Power): Doubling speed increases power by 8× (2³). This is because you're moving twice the air (Law 1) against four times the pressure (Law 2), requiring 8× the energy.
VFDs adjust motor speed electronically to match actual airflow demand. In VAV (Variable Air Volume) HVAC systems, fan speed is modulated based on duct static pressure, which responds to zone damper positions. At 60% airflow demand, the fan runs at ~60% speed, consuming only ~22% of full-speed power. Annual energy savings typically pay back VFD installation cost in 1-3 years.
Proper fan selection requires matching the fan curve to the system curve at the design operating point. Oversized fans waste energy running throttled or at low speed. The best efficiency point (BEP) should fall near the most common operating condition. AMCA (Air Movement and Control Association) certified ratings ensure published performance data is accurate.
Three relationships that predict fan performance changes with speed or diameter: (1) airflow ∝ speed, (2) pressure ∝ speed², (3) power ∝ speed³. They assume the system curve and fan efficiency remain constant.
Because power varies with the cube of speed, small speed reductions yield large power savings. Reducing fan speed by 20% cuts power by 49%. A VFD on a 10 HP fan running at 80% saves ~2.4 HP continuously, potentially $1,500+/year in electricity.
Fan laws are approximate when: (1) the system curve changes (e.g., dampers reposition), (2) the fan operates near stall/surge, (3) the speed change is very large (>±30%), or (4) Reynolds number effects become significant at very low speeds.
Yes, the same ratios apply with D₂/D₁ instead of N₂/N₁, but with an additional D ratio factor: Q ∝ D³, P ∝ D², HP ∝ D⁵ (when speed is held constant). This is used when resheaving or swapping wheels.
The system curve (P ∝ Q²) plots the pressure needed to push airflow through ductwork. The fan operating point is where the fan curve intersects the system curve. Fan laws slide the fan curve up/down along the system curve.
Measure airflow with a pitot tube traverse or flow hood at the duct. Measure static pressure with a manometer at the fan inlet/outlet. Measure power with a power meter at the motor. Calculate efficiency as (CFM × SP) / (6356 × BHP).