Calculate fan efficiency from flow rate, total pressure and power input, or find the missing variable in CFM, L/s, in. wg, psi, Pa, hp or kW.

Compare air power with fan input power.

Low-compressibility estimate. Shaft input gives fan efficiency; electrical input includes motor/drive losses.

Pressure increase across the fan, not absolute pressure.

Fan Efficiency Formula

The following low-compressibility estimate uses volume flow and total pressure rise across the fan at the same operating point. Conventional inches of water and mechanical horsepower are used.

Efan = FR ร— Pt / (6343.3115 ร— Pi) ร— 100
  • Where Efan is the Fan Efficiency (%)
  • FR is the fan flow rate (CFM) 
  • Pt is the total pressure (in. wg) 
  • Pi is the fan input power (mechanical hp); shaft power gives fan efficiency, while electrical input includes motor and drive losses 

To calculate the Fan Efficiency, multiply the fan flow rate by the total pressure, then divide by the product of 6343.3115 times the power input, then multiply by 100.

How to Calculate Fan Efficiency?

The following example problems outline how to calculate the Fan Efficiency.

Example Problem #1

  1. First, determine the fan flow rate (CFM). In this example, the fan flow rate (CFM) is given as 400.
  2. Next, determine the total pressure (in. wg). For this problem, the total pressure (in. wg) is given as  30.
  3. Next, determine the fan power input (hp). In this case, the fan power input (hp) is found to be 20.
  4. Finally, calculate the Fan Efficiency using the formula above: 

Efan = FR * Pt/ (6343.3115 * Pi) * 100

Inserting the values from above and solving yields: 

Efan = 400 * 30/ (6343.3115 * 20) *100 = 9.46 (%)

Example Problem #2

Using the same method as above, determine the variables required by the equation. For this example problem, these are provided as:

fan flow rate (CFM) = 500

total pressure (in. wg) = 30

fan power input (hp) = 10

Entering these given values yields: 

Efan = 500 * 30/ (6343.3115 * 10)*100 = 23.647 (%)