Vulnerability reduction to tsunamis has become a major issue after the December 2004 Indian Ocean tsunami disaster. An ex ante (before the event) evaluation of possible disaster reduction measures requires insight into the potential risks. As part of a study focusing on sea defence measures for Aceh and Nias provinces in Indonesia, we have developed a model that is capable of quantifying potential damages and casualties for tsunami-prone coastal areas. The model was able to reproduce the damage of 2004 sustained in Banda Aceh quite accurately. Because it is GIS based, the model also shows the spatial distribution of damage. Maps can be prepared that show high- and low-impact areas for various tsunami event scenarios. This information is very useful for cost–benefit analysis of mitigation measures, such as sea defence measures, spatial planning and evacuation procedures.