This paper describes the mathematical formulations that have underpinned the development of a strategic-level, personnel sustainability planning tool for the Australian Army. The tool considers personnel sustainability in terms of the dynamics of progression through career profiles and the requirements of the force to meet operational capability demands. It assists in analysing the ability of a force structure (current or future) to meet forecast operational deployments, providing insights into: likely problem areas with regard to sustainability, the effect of changing personnel policies, resource/cost implications and force expansion issues. Utilised in shorter-term planning, the tool allows for rapid force options testing and, in longer-term planning, for detailed sustainability analysis of proposed future force structures. The paper focuses on the basic construction of the model and its mathematical underpinnings, while giving insights into its utility and to how it has been applied.