The benefit (or additionality) attributable to a conservation action is the difference between the outcomes of two scenarios: (1) the scenario with the conservation action, and (2) the alternative scenario, in which action did not occur. However, many conservation decisions are made using approaches that do not appropriately calculate this benefit. We review recent scientific literature and conservation policies to examine how conservation benefit is calculated in three situations: systematic reserve selection, investment in agri-environment schemes, and biodiversity offset trades. In the examples we considered, the approaches used to calculate conservation benefit often involved assumptions about the alternative scenario that were not explicit, demonstrably wrong or both. We suggest that assumptions about how conservation value changes over time in the alternative scenario can often be substantially refined, and that making these assumptions explicit by calculating directly the expected difference between the two scenarios is likely to improve the quality of conservation decision-making.