In this paper, a software cost model with a warranty period and cost, a cost to remove each error detected in the software and a risk cost due to software failure is developed. A software reliability model based on the nonhomogeneous Poisson process is used. The optimal release policies which minimize the expected total software costs are presented. A software tool is also developed using Excel and Visual Basic that facilitates the task of determining the optimal software release time. Numerical examples are provided to illustrate the results.