In this paper, the influence of tip clearance on the performance of small-scale turbopumps is studied numerically on extensive parameter ranges suitable for organic Rankine cycle applications. A novel and fully parameterized design model is developed and used to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage. Impellers are investigated at different operating conditions, and the accomplished results are analyzed to characterize the performance at design and off-design conditions. The CFD calculations demonstrate that the slip factor is dependent not only on its geometrical parameters, as considered by most correlations, but also on its operating conditions. The slip factor decreased almost linearly as the flow rate increased. In addition, the tip clearance ratio influences the slip factor, and its influence is non-linear. The head rise decreases as the tip clearance ratios increase. However, for radial impellers, a higher head rise was observed for small tip clearance ratios (<0.10). The numerical data is employed to infer reduced-order models for considering the tip clearance effect in the early-phase design process of small-scale turbopumps. The models predict the CFD data with an average relative deviation of 3.6% and 5.8% for the slip factor and the head loss coefficient, respectively.}