Twenty-five years of genetic assessment of Symbiodinium diversity has revealed nine clades circumscribing more than 400 phylotypes, and fostered a nascent understanding of subphylotype diversity and population genetics. This diversity is meaningful for their symbiotic associations with corals, as the subset of phylotypes that have been assessed may differ in their physiological capabilities to acquire nutrients, produce photosynthetic products, and resist oxidative and thermal stress. These physiological differences translate into differential bleaching susceptibility of the holobiont and allow corals associated with thermotolerant phylotypes to persist under thermal conditions that would otherwise cause bleaching. This increased resistance is often associated with a tradeoff that may limit the optimal frequency of association with thermotolerant phylotypes to periods of thermal stress. Although the effects of Symbiodinium thermotolerance on bleaching susceptibility is a central focus of ongoing research, our ability to apply these data to conservation or remediative efforts is limited by the scope of comparable information across phylotypes. Here we present a novel quantitative index of thermotolerance for 110 Symbiodinium phylotypes based on an iterative partial-rank aggregation method of consensus estimation from available qualitative partial rankings. This new framework for analysis will allow quantitative assessment of hypotheses on the effects of Symbiodinium thermotolerance on coral bleaching.