AIM: To evaluate the effect of different lens constant optimization methods on the accuracy of intraocular lens (IOL) power calculation formulas for highly myopic eyes. METHODS: This study comprised 108 eyes of 94 consecutive patients with axial length (AL) over 26 mm undergoing phacoemulsification and implantation of a Rayner (Hove, UK) 920H IOL. Formulas were evaluated using the following lens constants: manufacturer’s lens constant, User Group for Laser Interference Biometry (ULIB) constant, and optimized constant for long eyes. Results were compared with Barrett Universal II formula, original Wang-Koch AL adjustment method, and modified Wang-Koch AL adjustment method. The outcomes assessed were mean absolute error (MAE) and percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 diopter (D). The nonparametric method, Friedman test, was used to compare MAE performance among constants. RESULTS: Optimized constants could significantly reduce the MAE of SRK/T, Hoffer Q, and Holladay 1 formulas compared with manufacturer’s lens constant, whereas the percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 D had no statistically significant differences. Optimized lens constant for long eyes alone showed non-significant refractive advantages over the ULIB constant. Barrett Universal II formula and formulas with AL adjustment showed significantly higher accuracy in highly myopic eyes (P<0.001). CONCLUSION: Lens constant optimization for the subset of long eyes reduces the refractive error only to a limited extent for highly myopic eyes.