Fitness is one of the central conceptual constructs of evolutionary biology, but its definition and quantification has remained controversial since its introduction to mathematical population genetics in the early 20th century. In this contribution I will propose a definition of fitness inspired by formal utility theory based on pair comparison systems. The resulting metrization establishes that fitness in non-overlapping generations is a ratio scale quantity. This fact is important for preventing artifacts in the detection of genotype by environment interaction. Finally I will present preliminary data from experiments with bacteria that illustrate the application of this approach to data gathering and analysis.