The Marine Corps’ manpower costs are 60 percent of its annual budget. The Enlisted and Officer Strength Planners must develop plans, by paygrade and month, to meet endstrength requirements in the budget execution year and 6 out-years. To develop these plans, the planners must forecast endstrength losses and gains. This study focuses on doing this accurately. Inaccuracy results in finishing either the year above the congressionally mandated endstrength target (overspending the budget) or below the endstrength target (which has operational consequences). Previously, there was no institutionalized and documented methodology for forecasting losses and no systematic attempt to improve existing techniques. New planners relied on information gleaned during overlap with their predecessors and sometimes developed their own methods (which were susceptible to errors). They had few reference tools and no capability to run loss scenarios. We first assessed the existing loss forecasting processes. Then, we made the processes more systematic. Next, we improved/added to the loss forecasting model and created reference tools. Finally, we documented the entire process in detail. We recommend creating an SSN-based file, adding a civilian planner/consultant to the endstrength team, and waiting to hard-wire models until the planners are comfortable with the modified models and their methods.