Engineers always need convenient calculation tools on the field or at the office for evaluating hydraulic characteristics and designing solutions while researchers develop scientific software in most cases nonreusable, invisible, and in the worst case, faulty and unreliable. Funded by the French Biodiversity Office, "Cassiopée" software has first been developed for the design and the verification of upstream and downstream fish passage facilities aggregating multiple tools required for dimensioning: pool-type fishways, pre-barrages, baffle fishways, nature-like fishways, and fish-friendly intakes. "Cassiopée" also includes numerous tools for open flow and pipe flow hydraulics for agriculture and environment. Based on calculation modules, which can be independent or linked in order to carry out complex operations, "Cassiopée" allows an iterative design approach ranging from the definition of the geometric characteristics of the devices to hydraulic simulations of their operating conditions. In a simple and user-friendly interface, it presents the calculation results as clear tables and graphs. Contextual documentation provides detailed information on the different tools and hydraulic formulas used for the calculations. Available in French and English, "Cassiopée" is an open source and free software accessible online (https://cassiopee.g-eau.fr) or as an offline application for Windows, Linux, macOS and Android. An R package is currently under development for advanced users who need to perform intensive calculations without a graphical user interface. Designed as a practical tool resulting from the transfer of research products, it is intended to be widely used by the hydraulic engineering community as a working and educational tool.