Structural and energetic properties for the lowest energy singlet and triplet states of the 19 possible didehydroindene isomers are predicted using coupled cluster, density functional, and multireference second-order perturbation theories. Singlet-triplet splittings and biradical stabilization energies provide a measure of the degree of interaction between the biradical centers. Comparisons to analogous didehydronaphthalenes are made to understand the influence of the five-membered ring. As in other didehydroarenes, proton hyperfine splittings in antecedent monoradicals are economical predictors of biradical state energy splittings.