The rate at which ions are transported from the body to the nonthermal tail of the distribution function during the nonlinear stage of the ion acoustic instability is calculated self‐consistently. The nonlinear stage is characterized by a very small growth rate and a tail whose population increases slowly in time. The calculation is done by conserving the energy and momentum of ions, electrons, and electrostatic fields in the linear and nonlinear regime. These two conservation laws give rise to two equations for growth rate and phase velocity.