Demographic data for most of the history of the Ottoman Empire is not quite precise. For most of the five centuries of its existence, the empire did not have easily computable valid data except figures for the number of employed citizens. Until the first official census (1881–1893), data was derived from extending the taxation values to the total population. Because of the use of taxation data to infer population size, detailed data for numerous Ottoman urban centers - towns with more than 5,000 inhabitants - is accurate. This data was collaborated with data on wages and prices. Another source was used for the numbers of landlords of households in the Ottoman Empire- every household was assumed to have 5 residents.
Town population in the Balkans in the 16th century
The first official census (1881–1893) took 10 years to finish. In 1893 the results were compiled and presented. This census is the first modern, general and standardized census accomplished not for taxation nor for military purposes, but to acquire demographic data. The population was divided into ethno-religious and gender characteristics. Numbers of both male and female subjects are given in ethno-religious categories including Muslims, Greeks (including Greek Macedonians, Asia Minor Greeks, Pontic Greeks, and Caucasus Greeks, all Orthodox Christians under the Greek Patriarchate from extremely distinct ethnic origin), Armenians, Bulgarians, Catholics, Jews, Protestants, Latins, Syriacs and Gypsies.
In 1867 the Council of States took charge of drawing population tables, increasing the precision of population records. They introduced new measures of recording population counts in 1874. This led to the establishment of a General Population Administration, attached to the Ministry of Interior in 1881-1882. Somehow, these changes politicized the population counts.
Population maps 1893-96
1893-96, Muslim, Greek and Armenian population
1893-96, Armenian distribution (in color)
1893-96, Green shows Muslim majority, red shows Armenian majority.
After 1893 the Ottoman Empire established a statistics authority (Istatistik-i Umumi Idaresi) under which results of another official census was published in 1899.
Istatistik-i Umumi Idaresi conducted a new census survey for which field work lasted two years (1905–06). 2-3 million people in Iraq and Syria remained unregistered and uncounted. As a factual note this survey's complete (total) documentation was not published. Results of regional studies on this data were published later, which were sorted by their publication date. Included in the publication and subsequent ones was the Ottoman Empire's population as of 1911, 1912, and 1914. The substantial archival documentation on the census has been used in many modern studies and international publications. After 1906 the Ottoman Empire began to disband and a chain of violent wars such as the Italo-Turkish War, Balkan Wars and World War I drastically changed the region, its borders, and its demographics.
Population distribution of the Millets in the Ottoman Empire in 1906, according to the official census-
Notes: a The Muslim Millet includes all Muslims. The largest of them being Turks, Arabs and Kurds. b The Greek Millet includes all Christians part of the Greek Orthodox Church. This includes Slavs and Albanians. c This includes the various Assyrian Churches. d The first source doesn't include Protestants and "others".
In 1865, 658600 (40,51%) Muslims and 967058 (59,49%) non-Muslims, including females, were living in the province excluding Niş sanjak and 569.868 (34,68%) Muslims, apart from the immigrants and 1.073.496 (65,32%) non-Muslims in 1859-1860. Half the Muslims were refugees from a population exchange of Christians and Muslims with Russia. Before the establishment of the Danube Vilayet, some 250000-300000 Muslim immigrants from Crimea and Caucasus had been settled in this region from 1855 to 1864. Another 200-300,000 male and female Circassian and Crimean Tatar refugees settled in 1862-1878 were to a degree excluded from the 1866 census count.
According to the 1874 census,there were 963596 (42,22%) Muslims and 1318506 (57,78%) non-Muslims in the Danube Province excluding Nış sanjak. Together with the sanjak of Nish the population consisted of 1055650 (40,68%) Muslims and 1539278 (59,32%) non-Muslims in 1874. Muslims were the majority in the sanjaks of Rusçuk, Varna and Tulça, while the non-Muslims were in majority in the rest of the sanjaks.
Ethnoreligious estimates and registered population
Ethnoreligious makeup of the vilayets in 1877
Until the census of 1881/82, although there were occasional allusions to ethnic groups in the 1831 census, Ottoman officials classified the inhabitants only according to religious affiliation, excluding some ethnic categories included in some of the provinces after 1868. Nationality was defined by religion rather than language. Sofia and Niş sancaks were separated from the Danube Vilayet and the detached Sofia Province was founded in 1876, which were annexed to Adrianople and Kosovo Vilayets respectively in 1877. Niş sanjak was part of Prizren Vilayet in 1869-1874.
Estimates in some eighteen sources show that the Muslims constituted about 35% of the total Balkan population during most of the first half of the 19th century, while in the second half of the century the proportion grew to 43%.  According to thirty three sources the figures given for the proportion of Turks in the European provinces range from 11 to 24% making up two thirds of the Muslims in the Danube Vilayet and most of them in the Adrianople Vilayet and Salonika Vilayet; for Bulgarians the figures are 24-39% with about 65 percent of the Christians in the Adrianople Vilayet and over 90 in the Danube Vilayet; for Greeks, from 9 to 16%; the rest were mainly Albanians. In Anatolia the absolute majority constituted the Turks. In the western European vilayets the Muslims were usually Slavs and Albanians. In the Ioannina Vilayet the Orthodox Christians were dominant, a majority of whom were ethnically Albanian according to Ottoman officials and were also three fourths of the Muslims. The Circassians are estimated at 1 million, of whom 400,000 in Asia Minor and 600,000 in Europe.
During the World War I; The treatment of Armenians in the Ottoman Empire was a book by Viscount Bryce and Arnold J. Toynbee which compiled statements from eyewitnesses from other countries including Germany, Italy, the Netherlands, Sweden, and Switzerland, who similarly attested to Armenians in the Ottoman Empire during 1915-1916. The publication presents Arnold J. Toynbee's analysis on Armenian population in the Ottoman Empire. A summary table of his analysis included in the page 199. In the "vilayet of Van", there were two portions, portions in modern use corresponds to county. As explained by Arnold J. Toynbee in the footprint at page 199, he developed his analysis by excluding certain portions of the province where he said "Armenians were a minor". Arnold Toynbee in finding the ratio of Armenians in vilayet of Van; he removed the values originating from portions of Van (listed in the foot print) where Armenians were in minority. The presented table in page 1999 shows the re-calculated values by Arnold J. Toynbee of these selected provinces using values of the parts (counties, sanjacks) which Armenians were not in minority. The presented map shows the re-calculated values of the stated provinces using values where Armenians are not in minority.
Ethnic values of the Six vilayets according to presented data.
^Behar, Cem, ed. 1996. Osmanlı Đmparatorluğu'nun ve Türkiye'nin nüfusu, 1500-1927. Ankara: T.C. Basbakanlık Devlet Đstatistik Enstitüsü = State Institute of Statistics Prime Ministry Republic of Turkey.
^Karpat, K.H. (1985). Ottoman population, 1830-1914: demographic and social characteristics. Madison, Wis: University of Wisconsin Pres.