The formula to calculate the new Fan Horsepower (HP2) is:

\[ HP2 = HP1 \left( \frac{D2}{D1} \right)^5 \left( \frac{N2}{N1} \right)^3 \]

Where:

• \(HP1\) is the initial fan horsepower (hp).
• \(HP2\) is the new fan horsepower (hp).
• \(D1\) is the initial fan diameter (in.).
• \(D2\) is the new fan diameter (in.).
• \(N1\) is the initial impeller speed (rpm).
• \(N2\) is the new impeller speed (rpm).