Abstract The graphical calculator originally developed for calculating penicillin concentrations from zone diameters in the three‐hour cylinder‐plate assay is shown to possess properties that recommend it for statistical analysis. By the introduction of two additional scales, the instrument is fitted to test data for deviation from normal distribution and to determine the mean and standard deviation from the mean of normally distributed values. Examples are included to demonstrate the advantages of graphical statistical analysis. A device for the semiautomatic plotting of data may be added if desired. The instrument is also useful in calculating values of LD 50 from suitable data.