While no master list of those who perished in the Holocaust exists anywhere in the world, since the 1940s, scholars, governmental agencies and Jewish organizations have consistently estimated the number of Jews murdered by the Nazis at around six million.
They have done so based on different records such as pre-World War II census reports, German archives and archives of other Axis countries, concentration camp records that were not destroyed by the Germans, wartime reports generated by officials responsible for implementing Nazi population policy, postwar demographic studies of population loss during the war, and post-war investigations.
While the Germans compiled statistics of Jews killed in 1942 and 1943, they ceased doing so during the last year and a half of the war. The Germans also destroyed many of the camp records when they realised they were losing the war. Their habit of producing documents in multiple copies has, however, allowed researchers to construct estimates of the numbers deported to some camps.
We will never, however, be able to more than estimate the true number of those who died, since we have no way of checking the German lists, or accounting for births and deaths that occurred after deportation. The shelves of the Hall of Names at Yad Vashem contain four million pages of testimony in which survivors and families have contributed information, but for those who were never known, there can be no record.
Today, there is still a considerable amount of research being carried out by experts at institutes such as the the US Holocaust Memorial Museum and Yad Vashem with the goal of filing and digitizing Nazi documents that detail the scope of their crimes.