Damage caused by floods can sometimes be catastrophic. When focusing on lessening the fatalities/victims, knowledge of a flood even just a day in advance could make a difference. Therefore, this research aims to develop a global real time calculation system for flood affected population. A new river model, CaMa-Flood, is introduced into the framework of Today's Earth, a real time calculation system for various hydrological aspects. Gridded population data was then combined to calculate the number of estimated affected population. Validations for the outputs of this system were conducted using multiple observation data. Comparison with floods in 2013 showed that this system was able to detect the occurrence of floods to a certain degree.