The historian Hernando del Pulgar, contemporary of Ferdinand and Isabella, estimated that the Inquisition had burned at the stake 2,000 people and reconciled another 15,000 by 1490 (just one decade after the inquisition began).
The first quantitative estimates of the number processed and executed by the Spanish Inquisition were offered by Juan Antonio Llorente, who was the general secretary of the Inquisition from 1789 to 1801 and published, in 1822 in Paris his Historia critica de la Inquisición. According to Llorente, over the course of its history, the Inquisition processed a total of 341,021 people, of whom at least 10% (31,912) were executed. He wrote, "To calculate the number of victims of the Inquisition is the same as demonstrating, in practice, one of the most powerful and effective causes of the depopulation of Spain." The principal modern historian of the Inquisition, Henry Charles Lea, author of History of the Inquisition of Spain, considered that these totals, not based on rigorous statistics, were very exaggerated.
Modern historians have begun to study the documentary records of the Inquisition. The archives of the Suprema, today held by the National Historical Archive of Spain (Archivo Histórico Nacional), conserves the annual relations of all processes between 1560 and 1700. This material provides information about 49,092 judgements, the latter studied by Gustav Henningsen and Jaime Contreras. These authors calculate that only 1.9% of those processed were burned at the stake.
The archives of the Suprema only provide information surrounding the processes prior to 1560. To study the processes themselves it is necessary to examine the archives of the local tribunals, however the majority have been lost to the devastation of war, the ravages of time or other events. Pierre Dedieu has studied those of Toledo, where 12,000 were judged for offenses related to heresy. Ricardo García Cárcel has analyzed thos of the tribunal of Valencia. These authors' investigations find that the Inquisition was most active in the period between 1480 and 1530, and that during this period the percentage condemned to death was much more significant than in the years studied by Henningsen and Contreras.
García Cárcel estimates that the total number processed by the Inquisition throughout its history was approximately 150,000. Applying the percentages of executions that appeared in the trials of 1560-1700--about 2%--the approximate total would be about 3,000 put to death. Nevertheless, very probably this total should be raised keeping in mind the data provided by Dedieu and García Cárcel for the tribunals of Toledo and Valencia, respectively. It is likely that the total would be between 3,000 and 5,000 executed. However, it is impossible to determine the precision of this total, owing to the gaps in documentation, unlikely that the exact number will ever be known.