The ship model test was believed to be one of the effective methods for figuring out the boundaries and reliability of the ship's horsepower. The ship's form factor determines a full-scale ship's effective horsepower. Determination of the form factor value can be done experimentally through the Prohaska method. The new method proposed in this study is employed the regression Iteratively Reweighted Least Squares (IRLS) method by utilizing the principle dimension of the ship, such as LWL, B, CB, CP, CM, WSA, T, ∆. etc. The Indonesian Hydrodynamics Laboratory has a database of ships with various principle dimensions which have undergone the towing model test. Through the database, the form factor can be predicted with the IRLS method. The method is then verified and validated with the Prohaska method. The result shows a good agreement with the Prohaska method. The obtained results from the IRLS method also show that the EHP & Resistance calculations are identical with old fashion Prohaska methods. The residual bias factor established by the IRLS method was verified in comparison to the value of the form factor generated by the Prohaska method. Comparison between the two methods results in a small error.