Background: Creep-fatigue damage occurs under cyclic loading at elevated temperature. The existing creep-fatigue models have limited ability to cover the full combination of creep and fatigue behaviours, except with extensive prior empirical testing. Consequently, they cannot effectively and efficiently be used for early engineering design. Approach: We present a strain-based unified creep-fatigue formulation that overcomes these limitations. We validate this equation against empirical data for multiple materials, and shows it is able to cover full ranges from pure fatigue to pure creep. A simplified formulation is developed where the coefficients are extracted through simple creep-rupture tests. We show how the equation may be used in a design situation, by application to a representative gas turbine blisk. Included here is a demonstration of how the equation may be integrated into finite element analysis, which is an important practical consideration in the design work flow. Outcomes: The results demonstrate that the unified equation evidences fidelity to empirical data for creep-fatigue behaviours for multiple metallic materials. The usefulness of this equation is the ability to identify candidate materials for creep-fatigue loading situations. The ability to achieve this at relatively early design stages is advantageous because of the economy and convenience provided.