A physics-based, reduced order model was developed to describe the capacity degradation in LiNiMnCoO2-graphite cells. By starting from fundamental principles, the model captures the effects of four degradation mechanisms: (i) SEI growth on the anode, (ii) electrolyte oxidation on the cathode, (iii) anode active material loss, and (iv) cathode active material loss, the last two due to chemomechanical fracture. The model is computationally efficient (∼1 ms/cycle) and enables physical, real-time, capacity loss calculations for automotive applications. Results demonstrate that under storage conditions, SEI growth and electrolyte oxidation are the major degradation mechanisms, in agreement with experiments. In contrast, batteries subjected to electric currents of a wide amplitude, close to the upper cutoff voltage, electrolyte oxidation contributes ∼50% of all the degradation mechanisms, consistent with recent experiments in the literature. Chemomechanically induced active material losses are maximal in the anode at high states of charge and maximal in the cathode at low states of charge. Results quantify the contribution to degradation from each individual mechanism, highlighting, for the first time, the need of physics-based, on-the-fly descriptions that go beyond traditional coulomb counting approaches. Finally, the identification of the individual degradation contributions enables the possibility of tailoring the charge/discharge sequence to extend battery life.