The Bolton Index is one of the most useful calculations for precise orthodontic diagnosis as it shows if there is a correct ratio between dental proportions. However, at times, this calculation is not applied because it is a long and time-consuming procedure compared with digital methods. A new digital method for measuring tooth sizes and for calculating the Anterior (ABI) and the Overall (OBI) Bolton Index was tested on 100 sets of study dental casts of the permanent dentition in a Spanish sample and compared with the traditional method. The reproducibility of this digital method versus the traditional one was analysed to determine intra- and inter-examiner measurement errors by calculating the coefficients of variation. The results demonstrated that the digital method provided results comparable with those of the traditional technique, since the regression parameters for each index showed that the correlation coefficients of the two methods were very high and similar to each other: r = 0.976 and r = 0.979 for the ABI and OBI, respectively. The results also showed more discrepancies in the ABI than in the OBI using both methods in this sample.