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Abstract Isospin symmetry is the most precise flavor symmetry. In this work, we propose an approach to generate isospin sum rules for heavy hadron decays without the Wigner-Eckhart invariants. The effective Hamiltonian of heavy quark weak decay is fully invariant under a series of isospin lowering operators I − n $$ {I}_{-}^n $$ and then the isospin sum rules can be generated through several master formulas. It provides a systematic way to study the isospin symmetry of c- and b-hadron weak decays. The theoretical framework of this approach is presented in detail with the nonleptonic decays of D and B mesons as examples. In addition, the V-/U-spin sum rules are derived in a similar algorithm by replacing I − n $$ {I}_{-}^n $$ with V − n / U − n $$ {V}_{-}^n/{U}_{-}^n $$ .