The exact number of people killed by the Nazi regime may never be known, but scholars, using a variety of methods of determining the death toll, have generally agreed upon common range of the number of victims. Recently declassified British and Soviet documents have indicated the total may be somewhat higher than previously believed. However, the following estimates are considered to be highly reliable. The estimates:
5.1–6.0 million Jews, including 3.0–3.5 million Polish Jews
1.8 –1.9 million non-Jewish Poles (includes all those killed in executions or those that died in prisons, labor, and concentration camps, as well as civilians killed in the 1939 invasion and the 1944 Warsaw Uprising)
500,000–1.2 million Serbs killed by Croat Nazis
200,000–800,000 Roma & Sinti
200,000–300,000 people with disabilities
80,000–200,000 Freemasons 
10,000–25,000 homosexual men
2,000 Jehovah's Witnesses
Raul Hilberg, in the third edition of his ground-breaking three-volume work, The Destruction of the European Jews, estimates that 5.1 million Jews died during the Holocaust. This figure includes "over 800,000" who died from "Ghettoization and general privation;" 1,400,000 who were killed in "Open-air shootings;" and "up to 2,900,000" who perished in camps. Hilberg estimates the death toll in Poland at "up to 3,000,000." Hilberg's numbers are generally considered to be a conservative estimate, as they generally include only those deaths for which some records are available, avoiding statistical adjustment. British historian Martin Gilbert used a similar approach in his Atlas of the Holocaust, but arrived at a number of 5.75 million Jewish victims, since he estimated higher numbers of Jews killed in Russia and other locations.
Map titled "Jewish Executions Carried Out by Einsatzgruppe A" from the December 1941 Jäger Report by the commander of a Nazi death squad. Marked "Secret Reich Matter," the map shows the number of Jews shot in the Baltic region, and reads at the bottom: "the estimated number of Jews still on hand is 128,000". Estonia is marked as judenfrei ("free of Jews").Lucy Davidowicz used pre-war census figures to estimate that 5.934 million Jews died. Using official census counts may cause an underestimate since many births and deaths were not recorded in small towns and villages. Another reason some consider her estimate too low is that many records were destroyed during the war. Her listing of deaths by country is available in the article about her book, The War Against the Jews.
One of the most authoritative German scholars of the Holocaust, Prof. Wolfgang Benz of the Technical University of Berlin, cites between 5.3 and 6.2 million Jews killed in Dimension des Volksmords (1991), while Yisrael Gutman and Robert Rozett estimate between 5.59 and 5.86 million Jewish victims in their Encyclopedia of the Holocaust (1990).
The following groups of people were also killed by the Nazi regime, but there is little evidence that the Nazis planned to systematically target them for genocide as was the case for the groups above.
3.5–6 million other Slavic civilians
2.5–4 million Soviet POWs
1–1.5 million political dissidents
Additionally, the Nazis' allies, the Ustaša regime in Croatia conducted its own campaign of mass extermination against the Serbs in the areas which it controlled, resulting in the deaths of at least 330,000–390,000 Serbs.