The formula to calculate the new Impeller Diameter (D2) is:

\[ D2 = D1 \left( \frac{Q2}{Q1} \right)^{1/2} \left( \frac{H1}{H2} \right)^{1/4} \]

Where:

• \(D1\) is the initial impeller diameter (in.).
• \(D2\) is the new impeller diameter (in.).
• \(Q1\) is the initial air flow rate (cfm).
• \(Q2\) is the new air flow rate (cfm).
• \(H1\) is the initial fan head (in. H2O).
• \(H2\) is the new fan head (in. H2O).