Determining the bubble wall velocity during a first-order phase transition is of great importance to accurately predict the resulting gravitational wave signals and matter-antimatter asymmetry. In this talk, I will explain how this calculation can be significantly simplified when local thermal equilibrium (LTE) is maintained in the plasma. Using this LTE assumption, I will show that the scalar fields' equations of motion can be replaced by a new matching condition which can be interpreted as the conservation of entropy. The resulting system of equations can be expressed in terms of only four parameters that completely characterize a particle physics model. I will present an efficient algorithm to solve these equations and discuss the properties of their solutions. Even when the LTE assumption is badly violated, these solutions can still be helpful as they provide an upper bound on the wall velocity. Finally, I will compute the kinetic energy fraction which is essential for predicting the gravitational wave spectrum produced during the phase transition.