By the early 20th century, the population of the Democratic Republic of the Congo was around 20 million under the rule of Leopold II of Belgium. Historians estimate that around 10 million people died between 1880 and 1920 due to famine, violence, disease, and social collapse.

“At this moment, nobody is in a position (1900–1904), in any way, even to estimate the number of inhabitants living in the Congo, let alone to determine the losses: ‘Historians are confronted with a total absence of reliable figures to count the indigenous population of the Congo.’ … Based on testimonies of colonists or missionaries… some allow themselves to launch figures that vary strongly… Britannica gives a population loss of 8 to 30 million… Jan Vansina… estimated that the population between 1880 and 1920 fell by 50%… The most reasonable hypothesis we have is that of Jean‑Paul Sanderson… who concludes that the Congolese population went from 15 million (highest estimate) or 10.5 million (lowest estimate) in 1885 to about 10 million in the 1920s – a reduction of at least half a million up to five million people.”

“Precisely determining how many people died as a result of Leopold’s rule over the Congo is one of the most debated issues in the historiography of the period. The difficulty is that no reliable population census exists for the Congo basin before Leopold’s acquisition, and the first systematic Belgian colonial census was not conducted until 1924, after the population had already been reduced by the worst of the violence and its aftermath. Contemporary estimates of the pre‑colonial Congolese population range from approximately ten million to thirty million… If these figures are approximately correct, the population of the Congo basin fell by between ten and fifteen million people during the period of Leopold’s rule and its immediate aftermath… Other scholars have offered lower estimates, ranging from one million to five million excess deaths attributable specifically to the violence and disruption of the Leopold era.”

“Estimating the extent of the population decrease during this period is difficult due to the lack of reliable demographic data. The population of the entire state is said to have declined from some 20 million to 8 million. Contemporary estimates of population loss are necessarily speculative, and historians disagree as to the exact numbers. The absence of systematic censuses before the 1920s makes precise calculations impossible.”

The aim of this thesis is to present a method to exploit the available data and show results in terms of mortality and in terms of evolution of the Congolese population. By using the properties of model life tables and tables of stable populations, we can use the data structure to rebuild life tables. It can cover a period from 1937 to 1956. By using backward-projection, we can estimate the evolution of the whole population from 1955 to 1885.

Finalement, pour les années antérieures à 1900, disons à partir de 1877, l’année de la conférence de Berlin, les données démographiques sont lacunaires et reposent sur des estimations. … Pour la période qui va de 1885 à 1920, J.-P. Sanderson conclut que la population congolaise est passée de 15 millions (estimation la plus élevée) ou de 10,5 millions (estimation la plus basse) en 1885 à environ 10 millions dans les années 1920 – soit une réduction d’au moins un demi-million jusqu’à cinq millions de personnes. Sanderson souligne cependant le caractère hypothétique de ces chiffres, qui dépendent de la qualité très inégale des sources disponibles avant les premiers recensements systématiques.

“In the absence of a census providing even an initial idea of the size of population of the region at the inception of the Congo Free State (the first was taken in 1924), it is impossible to quantify population changes in the period. … The current estimate for the population in 1885 is stated at around most likely 11.5 million. The first head count of the population after Leopold’s reign put the population at 10 or 10.3 million, which is a population decrease of around 1.2 or 1.5 million people. … Louis and Stengers state that population figures at the start of Leopold’s control are only ‘wild guesses’, while calling Morel’s attempt and others at coming to a figure for population losses ‘but figments of the imagination’.”

“Exact population figures at the beginning of Leopold II’s rule are not known; estimates, however, assume around 20 million inhabitants around 1885. During the colonial period no comprehensive census was carried out. Only in 1924, many years after the transfer to Belgium, did a count determine about 10 million Congolese. Adam Hochschild estimated that between 1880 and 1920 up to around 10 million Congolese died… Other estimates range from one to five million dead; exact numbers remain uncertain, since no reliable statistics exist.”

“In the absence of a census that provides even an initial idea of the size of the population of the region at the start of the Congo Free State (the first was carried out in 1924), it is impossible to quantify with high precision the changes in the population in the period. … Various historians question Casement’s figure due to the absence of reliable censuses, the enormous mortality from diseases such as smallpox or sleeping sickness, and the fact that in 1900 there were only 3,000 whites in the Congo, of whom only half were Belgians.”

The article points out that early population estimates for Central Africa were "likely exaggerated and unreliable" and that accusations against the Leopold regime citing up to 10 million lives lost between 1890-1910 were made "though census data was lacking." It notes that suggested figures for the Congo between 1885 and 1910 ranged from 20 to 30 million inhabitants, but that "given the lack of a reliable population census" observers assumed a 50% reduction using a hypothetical figure of 18 million in 1885, and that population historians "unanimously reject the high estimates for the 1890s."

The article states: "While it has proven difficult to accurately estimate the pre-colonial population and the extent to which it changed under the Congo Free State, estimates for the Congolese population decline during Leopold's rule range from 1 million to 15 million." It notes that several historians argue against the widely cited 10 million figure "due to the absence of reliable censuses" and quotes Louis and Stengers (1968) as saying that population figures at the start of Leopold's control are "only 'wild guesses'" and attempts to determine losses are "'but figments of the imagination.'"

No existe forma de evaluar con cierto grado de credibilidad el número de víctimas durante la existencia del Estado Libre. Las cifras solo pueden ser orientativas. El profesor Jan Vansina, de la Universidad de Wisconsin, antropólogo y etnógrafo que ha estudiado con detenimiento la población de la zona, afirmó que entre 1880 y 1920 la población del Congo se redujo «por lo menos a la mitad». This passage stresses that there is no way to assess with a credible degree the number of victims during the Congo Free State; figures can only be indicative, and Vansina’s "at least half" estimate is itself based on indirect reconstruction, not on a baseline census.

“1885–1908 Millions of Congolese died as a consequence of exploitation and disease, population declined dramatically and determining precisely how many people died is impossible, as no accurate records exist. No reliable nationwide census was conducted during the Congo Free State period; later Belgian colonial counts began only in the 1920s.”

“It is probably over many millions, possibly 10 million murdered or more from 1885… Britannica, ‘Congo Free State’ claims that the population declined from 20 or 30 million to 8 million. A 1904 report by Roger Casement estimated that as many as 3 million Congolese died since 1888… Peter Forbath… claims that at least 5 million killed… Adam Hochschild… estimates 10 million, or half the original population from 1885 to 1920… Fredric Wertham… estimates that the population of the Congo dropped from 30M to 8.5M, a loss of 21.5 million. None of these figures are based on an actual baseline census; all are reconstructions from partial reports and later counts.”

“Later academic research finds that during Leopold’s rule and its immediate aftermath, Congo’s population may have been slashed by as many as 10 million people. These estimates are based on comparative analysis of later colonial censuses, missionary records, and administrative reports, since no reliable population baseline exists for the Congo basin in 1885.”

*Inhabitants.*—The population is variously estimated at from 14,000,000 to 30,000,000. This 1911 Encyclopædia Britannica entry for the Congo Free State shows that even contemporary observers gave very wide-ranging estimates for the number of inhabitants, reflecting the absence of a precise census and reliance on rough guesswork for population size around the turn of the twentieth century.

This primary-source document is useful as a contemporaneous U.S. diplomatic record from the year 1885, but it does not provide a reliable census baseline for the Congo population; it only establishes the historical timeframe of Leopold II's Congo project.

Population data for the Democratic Republic of the Congo are shown only from the modern period onward in this profile, underscoring the lack of a continuous official baseline back to 1885.

“Indeed, it is now found that population decline from 1885 was nowhere near the numbers so long alleged but around 1–2 million – a vast difference and discrepancy to the thesis and numeric put out by Adam Hochschild… The widely cited claim that ‘ten million died’ is not derived from a pre‑existing census. No reliable population count existed before 1885. Estimates of demographic loss are reconstructed retrospectively from missionary reports, later colonial enumerations, tax records, and modelling. Depending on the assumed starting and end points, implied losses vary considerably… This… underlines the uncertainty embedded in the arithmetic.”

An AskHistorians contributor notes that "there just aren't meaningful population data for the period of Leopold's personal rule (1885-1908), still less for any time previously, and hence no sound basis for an estimate of total excess mortality inflicted under his exceptionally brutal regime." They explain that the common "10 million" figure appears to derive from late 19th-century European guesses of a 20 million population at a time "when there was simply no way of knowing" and conclude: "it remains impossible to put a reliable figure on the deaths."

This page presents historical population series for the Democratic Republic of the Congo, but such reconstructions are model-based and not direct censuses for the 1885 Congo Free State period.

“William Rubinstein wrote: ‘More basically, it appears almost certain that the population figures given by Hochschild are inaccurate. There is, of course, no way of ascertaining the population of the Congo before the twentieth century and estimates like 20 million are purely guesses. Most of the interior of the Congo was literally unexplored if not inaccessible.’ This criticism highlights that any estimate of Congo’s population in 1885 necessarily rests on conjecture rather than census data.”

The educational resource states that from 1885–1908 "millions of Congolese died as a consequence of exploitation and disease" and that in some areas populations declined dramatically. It reports that "a government commission later concluded that the population of the Congo was 'reduced by half' during this period, but no accurate records exist." This indicates that even official inquiries relied on broad approximations rather than precise population baselines.

The video description and transcript summarise analysis of demographic data suggesting that if one tried to estimate the Congo Free State's population around 1900 without assuming a prior "10 million killed" narrative, mid-range estimates would be around 7 million, with a plausible upper limit of roughly 10.7 million and an absolute maximum of 15.2 million, and a low estimate of 4 million with an absolute minimum of 3.1 million. The presenter stresses that these ranges are reconstructed and that the often-cited starting population of 20 million is not empirically grounded in a contemporaneous census.

A government commission later concluded that the population of the Congo was “reduced by half” during this period, but no accurate records exist. From 1885–1908, millions of Congolese died as a consequence of exploitation and disease. In some areas, the population declined dramatically; it has been estimated that sleeping sickness and smallpox killed nearly half the population in the areas surrounding the lower Congo River. The article notes that despite such dramatic statements, there were no accurate records, reinforcing that any estimate of total population loss for the Congo Free State era rests on indirect evidence rather than a firm demographic baseline.

“Historians are confronted with the absence of reliable figures for counting the indigenous population of the Congo at the turn of the century. Some contemporary reports, such as Roger Casement’s estimate of a three‑million population loss, are based on partial observations and are considered ‘almost certainly an underestimate’ by later scholars. A diversity of figures and the lack of any comprehensive survey mean that attempts to quantify the demographic impact of Leopold II’s rule involve significant uncertainty.”