Engineers and designers who select and specify fans use fan laws to accurately predict changes (assuming the fan diameter and air density are constant) in fan or blower performance. The following laws govern the performance specifications of Beckett Air blower wheels:
| When these conditions are fixed | and this is the variable | The laws are | The equations are |
| Impeller | Speed (n) | 1. Flow rate varies directly as the speed ratio. | Q2 = Q1 n2/n1 |
| System | 2. Pressure varies as the square of the speed ratio. | p2 = p1 (n2/n1)² | |
| Density | 3. Horsepower varies as the cube of the speed ratio | HP2 = HP1 (n2/n1)³ | |
| Speed | Impeller Diameter(D) | 4. Flow rate varies as the cube of the fan diameter ratio. | Q2 = Q1 (D2/D1)³ |
| Density | 5. Pressure varies as the square of the fan diameter ratio. | p2 = p1 (D2/D1)² | |
| Point of Rating | 6. Horsepower varies as the 5th power of the fan diameter ratio. | HP2 = HP1 (D2/D1)⁵ | |
| Flow Rate | Density (p) | 7. Horsepower and pressure vary directly as the air density ratio. | p2 = p1 (p2/p1) |
| Speed | HP2 = HP1 (p2/p1) |
Legend
n = Fan speed in revolutions per minute
Q = Flow rate in cubic feet per minute
p = Pressure in inches of water column
HP = Horsepower
D = Fan Diameter in inches
p = Air density in lbs./cubic foot (standard p, = .075 lbs./cu. ft.)
Subscript (1) denotes values for existing condition
Subscript (2) denotes values computed to the desired conditions