We propose a definition of public debt sustainability based on the possibility of conducting a fiscal effort or giving support to a macroeconomic path that makes it possible to reach a public debt target over a given horizon. The concepts of a fiscal effort and a macroeconomic trajectory are both speculative, as they rely on the anticipation of unknown futures. By making the parameters of these futures explicit and using them in a parsimonious model, we can generate trajectories that are not forecasts but a means of assessing the effort required to reach a target that is conditional on explicit assumptions. Debtwatch is a web application, freely accessible at https://ofce.shinyapps.io/debtwatchr, that can be used to carry out simulations, not only for France but also for other European countries and certain non-European countries such as the United States, including by modifying the parameters and exchanging assumptions with others. It is possible to carry out a calculation that is transparent (the assumptions are known and can be shared) and reproducible (the same assumptions lead to the same results) and which should help to further the debate on public debt targets and the associated efforts for a selection of developed countries.