Wetlands can absorb excess water in cases of river flooding. This method can be used to estimate the retention capacity of wetlands for flood mitigation and thereby protecting downstream inhabitants. For this, the impact of water detention on peak flows is considered, as well as the extent and frequency of flood events, and how flooding affects people and their properties. Hydrological data are necessary (inflow to and outflow from a wetland). Basic calculations of differences provide first approximations of wetland’s flood prevention potential. No modelling required. Maps show spatially explicit results.