As interest mounts in nature-based solutions (NBS) for flood mitigation as complementary options to civil-engineering measures, possible flood-protection strategies have become more diverse and hence complicated to assess. We offer a straightforward and educational protocol targeted for effectiveness analysis and decision making involving stakeholder participation. It is based on the concept of flood-excess volume (FEV), the volume exceeding a threshold and generating flood damage, and explores what fraction of FEV is reduced, and at what cost, by particular flood-mitigation measures. Quantification and interpretation of cost scenarios are facilitated using a graphical display that is easy to understand and encapsulates concepts of flood magnitude, FEV and protection-measures efficacy. It is exemplified for two recent extreme-flood events on the River Calder in Mytholmroyd (Yorkshire, United Kingdom) and the River Brague in Biot (Alpes-Maritimes, France). Each case has different flood-mitigation measures such as natural water-retention measures, tree planting, river-bed widening, or use of reservoirs and floods walls. Our straightforward protocol enables fast, quantifiable and easy-to-understand exploration of protection strategies using multiple measures, and in doing so highlights the issue of NBS scalability.