<p>In the pipe entrance region, the developing velocity and temperature profiles enhance heat transfer but increase pressure drop. This makes the transition to fully developed flow critical in the thermal-hydraulic design of molten salt systems. However, most existing correlations for low-temperature fluids were originally derived for hydrodynamically fully developed but thermally developing flows. This study numerically investigated the hydrodynamic and thermal entrance lengths for laminar developing flow of molten salt in horizontal pipes under a uniform heat flux condition, with a systematic analysis of the effects of key parameters, such as Reynolds number, pipe inner diameter, Prandtl number, inlet temperature, and heat flux. The results demonstrate that existing correlations show acceptable predictive deviation for the thermal entrance length but yield large underpredictions for the hydrodynamic entrance length under forced convection; the latter increases significantly due to the variation of viscosity along the flow direction. Meanwhile, existing correlations exhibit significant deviations for both lengths under mixed convection, as the classical entrance length criteria do not adequately capture the combined effects of buoyancy and temperature-dependent viscosity. In mixed convection, buoyancy forces shorten the thermal entrance length but extend the hydrodynamic entrance length, with radial viscosity variation playing a noticeable role. Finally, new correlations for hydrodynamic and thermal entrance lengths under both forced and mixed convection regimes were proposed, with a maximum relative deviation of 7.0% from the present numerical simulation data. However, the maximum relative uncertainty of mixed convection hydrodynamic entrance length correlation is 27.0% considering the sensitivity of the adopted criterion.</p>