Fan Calculator: Power, Airflow, Efficiency and Fan Laws
Enter any two of airflow, static pressure, and efficiency to get shaft power and brake horsepower. Switch to Fan Affinity Laws mode to predict how a fan's flow, pressure and power change when you change its speed. Results update instantly; choose your preferred flow and pressure units.
Fan power, airflow, and pressure: the core triangle
Every fan operating point sits at the intersection of two relationships: the fan curve (how much pressure it generates at each flow rate) and the system curve (how much pressure the ductwork, filters, and fittings demand at each flow rate). The fundamental power equation ties these together: Air Power (W) = Flow (m3/s) x Static Pressure (Pa). Because no fan is perfectly efficient, the motor must supply more than the air power: Shaft Power = Air Power / Efficiency. This calculator lets you solve for any one of the four linked quantities - flow, pressure, shaft power, or efficiency - when the other three are known.
Fan affinity laws: scaling with speed
The three fan affinity laws (also called fan similarity laws or AMCA 201 laws) describe how flow, pressure, and power scale when a fan's rotational speed changes while the system resistance stays fixed. Law 1: airflow is directly proportional to speed (Q2 = Q1 x N2/N1). Law 2: static pressure is proportional to the square of speed (dP2 = dP1 x (N2/N1)^2). Law 3: shaft power is proportional to the cube of speed (W2 = W1 x (N2/N1)^3). The cube law is the key insight behind variable-frequency drives (VFDs): reducing a fan to 80% speed cuts power consumption to about 51% (0.8^3 = 0.512), and reducing to 50% speed cuts power to just 12.5% of the original. These laws assume similar fan geometry, constant air density, and that you are comparing points on the same system curve.
How to read and use fan curves
A fan performance curve plots static pressure on the vertical axis against airflow on the horizontal axis. The fan curve slopes downward from left (high pressure, zero flow) to right (maximum flow, zero pressure). A separate power curve shows shaft power across the same flow range. The system curve is a parabola starting at the origin (zero flow, zero pressure losses) and rising steeply because duct friction scales with the square of velocity. The operating point is where fan curve and system curve intersect. Changing fan speed shifts the fan curve upward (higher speed) or downward (lower speed) while the system curve stays fixed; the affinity laws predict where the new intersection lands. Adding resistance - closing a damper, loading a filter - steepens the system curve and shifts the operating point left toward lower flow.
Selecting the right fan for your system
Centrifugal backward-curved fans achieve the highest total efficiencies (70-85%) and are the standard choice for air handling units, laboratory exhaust, and clean commercial HVAC. They are also non-overloading: power consumption peaks near the design point and actually falls at higher-than-design flows, preventing motor burnout. Forward-curved fans are compact and inexpensive but less efficient and can overload the motor at low static pressure. Axial and vane-axial fans excel at high flow with low resistance and dominate roof ventilation and cooling tower applications. For system design, calculate total equivalent resistance for all duct runs and fittings, select a fan whose curve intersects the system curve at least 20% into the high-efficiency region, size the motor with a 15% service factor, and plan duct connections at least three equivalent diameters from any turn to avoid system-effect losses.
Typical fan efficiencies and pressure ranges by fan type
| Fan type | Total efficiency (%) | Typical static pressure | Common application |
|---|---|---|---|
| Axial (propeller) | 40-60 | 0-500 Pa / 0-2 in H2O | Low-resistance general ventilation |
| Axial (vane-axial) | 55-70 | 0-800 Pa / 0-3.2 in H2O | HVAC supply/return, roof fans |
| Centrifugal (backward-curved) | 70-85 | 250-5000 Pa / 1-20 in H2O | Air handling units, clean air |
| Centrifugal (forward-curved) | 55-65 | 200-3000 Pa / 0.8-12 in H2O | Residential furnaces, FCUs |
| Centrifugal (radial blade) | 50-65 | 1000-8000 Pa / 4-32 in H2O | High-pressure industrial |
| Mixed flow | 60-75 | 200-2000 Pa / 0.8-8 in H2O | In-line HVAC, data centers |
| Cross flow | 30-50 | 50-300 Pa / 0.2-1.2 in H2O | Fan coil units, ductless splits |
General guidance from ASHRAE and AMCA for common fan types. Always verify with manufacturer data.
Frequently asked questions
What is the difference between static pressure and total pressure in a fan system?
Static pressure acts equally in all directions and is what the fan must overcome to push air through ducts, filters and fittings. Velocity pressure is the kinetic energy of the moving air. Total pressure is the sum of the two. When engineers size fans for duct systems they normally work with total static pressure (often called fan static pressure), which equals the rise in total pressure across the fan minus the velocity pressure at the fan outlet. Most fan performance curves are plotted using total static pressure.
How does air density affect fan performance?
Fan airflow (CFM or m3/s) is largely unchanged by air density at a given speed, but pressure and power are directly proportional to density. At altitude or high temperature, where air is less dense, a fan produces proportionally less static pressure and requires less shaft power. To compensate for altitude you can increase fan speed using the affinity laws, or select a larger fan. The reference density is 1.2 kg/m3 (dry air at 20 degrees C and sea level). A correction factor of actual density / 1.2 scales pressure and power results.
What is brake horsepower (BHP) and why does it matter?
Brake horsepower is the mechanical power delivered to the fan shaft, measured (historically) by a prony brake at the motor output. It equals shaft power in watts divided by 745.7. BHP matters for motor selection: a motor nameplate rated at 1 HP can supply 745.7 W to the shaft continuously. Always include a service factor of 1.10 to 1.15 when sizing the motor so that system variations, dirty filters and belt losses do not cause overheating.
Can I apply the fan affinity laws to any speed change?
The affinity laws give accurate predictions as long as the fan geometry stays the same, air density is constant, and the speed change keeps the operating point in a similar region of the fan curve (turbulent, attached flow). They break down at very low speeds where flow becomes laminar, and they do not account for motor slip, belt losses, or the fact that fan efficiency is not constant across the full speed range. For large speed changes (more than about 20-30%) confirm the new operating point against the manufacturer's full family of performance curves.
What fan efficiency should I design for?
Target the highest efficiency that fits your budget and space constraints. For new commercial HVAC, ASHRAE 90.1 sets a minimum Fan Energy Index (FEI) of 1.00 for most applications, which roughly corresponds to a fan total efficiency of 55-65% depending on fan class. High-efficiency backward-curved centrifugal fans in well-designed systems routinely achieve 70-80%. Avoid selecting a fan at the far left (near shut-off) or far right (near free delivery) of its curve, where efficiency falls sharply and the operating point can become unstable.
How much energy can a VFD save on a fan application?
Because power scales with the cube of speed, reducing fan speed by 20% (speed ratio 0.8) cuts power to 51% of the original - a 49% saving. Reducing speed by 40% (to 60% of original) cuts power to just 22%. In variable-air-volume (VAV) HVAC systems where demand fluctuates, VFDs are almost always cost-effective for motors above about 5 kW. Payback periods of one to three years are common in commercial buildings.