In this work, we present a quantitative model to understand the interplay between delocalization, strain energy and reactivity (Figure ). We propose that enhanced electronic delocalization within three-membered rings results in earlier, lower energy TSs, an effect that is distinct from barrier lowering due to strain release alone. This model not only accounts for the relative reactivity of cyclopropane and cyclobutane, but extends to all molecules containing one or more three-membered rings, including heterocycles and polycyclic structures. We demonstrate that in many cases, delocalization is the primary factor that governs reactivity, for example in ringopening reactions of bicyclo[1.1.0]butanes, [1.1.1]propellane, and epoxides. We show that this principle can be generalized in a simple 'rule-of-thumb' in which each three-membered ring fused to the breaking bond lowers the activation barrier by ~10 kcal mol -1 , corresponding to a ~10 fold rate enhancement at 298 K. This model can also be applied to 'strain-promoted' azidecycloalkyne (3+2) cycloadditions, which are commonly used as a bioconjugation strategy. Collectively, this framework unites the influence of strain-driven and delocalization-enabled reactivity, and offers quantitative predictions of reaction barriers.