Station observation is a good data source to monitor the potential evapotranspiration (PET) of a specific site particularly for the purpose of crop irrigation activities; however it represents only the site geographic characteristics and provides real time or historical records. Hence, there was an urgent need to find a promising tool and a simple empirical to predict/project the PET in locations where station observation is not available. The Hargreaves–Samani method (HS) method is recommended after the Penman-Monteith equation. Also, it can operate on daily and monthly time scales. To handle this issue, the regional climate model (RegCM4) was used to compute the PET using the HS. ERA-Interim reanalysis of 1.5 degrees (EIN15) was used to downscale the RegCM4 with 25 km grid spacing over Egypt for the period 1979-2017. A comparison was conducted between the original HS formula and its calibrated version with respect to the high-resolution ERA5-land derived reanalysis product (hereafter ERA5). Results showed that calibrating the HS coefficients indicates a considerable improvement in estimating the PET (relative to the original equation) when it is compared with ERA5. Such improvement is confirmed by a significant low mean bias. Therefore, the RegCM4 can be used to estimate the PET using a calibrated version of the HS equation either for making a daily forecast (e.g., for 5 days in advance) or projecting the future PET under different global warming scenarios.