The unprecedented growth of cities has a significant impact on future flood risk that might exceed the impacts of climate change in many metropolitan areas across the world. Although the effects of urbanisation on flood risk are well understood, assessments that include spatially explicit future growth projections are limited. This comparative study provides insight in the long term development of future riverine and pluvial flood risk for 18 fast growing megacities. For these cities a spatially explicit urban growth model has been developed capable to identify and extrapolate spatial development trends into growth projections for the short, medium and long term. For some cities like Dhaka or Lahore, the outcomes are alarming while others show an implicit tendency for flood adverse urban growth. The outcomes not only provide a baseline absent in current practise, but also a strategic outlook that might better establish the role of urban planning in limiting future flood risk.