Beyond reasonable doubt represents a probability value that acts as the criterion for conviction in criminal trials. I introduce the membership function (MF) method as a new tool for measuring quantitative interpretations of reasonable doubt. Experiment 1 demonstrated that three different methods (i.e., direct rating, decision theory based, and MF) provided significantly different and uncorrelated interpretations of reasonable doubt, although all methods predicted verdicts equally well, and showed inter-individual variability in interpretations. In Experiment 2 only the direct rating method demonstrated a significant effect of judicial instructions on reasonable doubt. In both experiments, the MF method showed intra-individual variability in interpretations of reasonable doubt. The methods may be capturing different aspects of the concept of reasonable doubt. These findings have implications for the validity of past research findings on reasonable doubt and for the utility of triangulation of methods in future research.