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In 1981, Sansuc obtained a formula for Tamagawa numbers of reductive groups over number fields, modulo some then unknown results on the arithmetic of simply connected groups which have since been proven, particularly Weil's conjecture on Tamagawa numbers over number fields. The same formula quickly generalizes to all linear algebraic groups over number fields. Sansuc's method also works for reductive groups in the function field setting, thanks to the recent resolution of Weil's conjecture in the function field setting by Lurie and Gaitsgory. However, due to the imperfection of function fields, Sansuc's formula does not hold for all linear algebraic groups over function fields. We propose a modification of Sansuc's formula that recaptures it in the number field case and also gives a correct answer over function fields. We have proven this formula for all pseudo-reductive groups in characteristic greater than 3, as well as for all commutative groups (in any characteristic). The commutative case (which is essential even for the general pseudo-reductive case) is a corollary of a generalization of the Poitou-Tate nine-term exact sequence, from finite group schemes to arbitrary affine commutative group schemes of finite type.