We prove a generalization of Demailly-Ein-Lazarsfeld’s subadditivity formula [DEL] and Mustaţǎ’s summation formula [Mu] for multiplier ideals to the case of singular varieties, using characteristic p methods. As an application of our formula, we improve Hochster-Huneke’s result [HH4] on the growth of symbolic powers of ideals in singular affine algebras.