Radiative transfer models (RTMs) have been used to estimate grain size of amorphous and crystalline water (H2O) ice in the outer solar system from near-infrared (NIR) wavelengths. We use radiative scattering models to assess the discrepancy in grain size estimation of H2O ice at a temperature of 15, 40, 60, and 80 K (amorphous) and 20, 40, 60, and 80 K (crystalline)—relevant to the outer solar system. We compare the single scattering albedos of H2O ice phases using the Mie theory and Hapke approximation models from the optical constant at NIR wavelengths (1–5 μm). This study reveals that Hapke approximation models—Hapke slab and internal scattering model (ISM)—predict grain size of crystalline phase slightly closer to Mie model than amorphous phase at temperatures of 15–80 K. However, the Hapke slab model predicts, in general, grain sizes much closer to those of the Mie model's estimations while ISM predicted grain sizes exhibit a higher uncertainty. We recommend using the Mie model for unknown spectra of outer solar system bodies to estimate H2O ice grain sizes. While choosing the approximation model for employing RTMs, we recommend using a Hapke slab approximation model over the ISM.