You’re right in saying that almost every other estimate falls far below 20 million. It’s impossible to quantify the exact amount of people who died under King Leopold’s rule, especially when we’re not even sure how many people lived there. The first official census wasn’t undertaken until 1924 and that arrived at a figure of 10 million. When combined with Vansina’s claim that the population declined by half between 1880-1920, this gives us figure of 10 million deaths. You could add both numbers together to arrive at 20 million, but that’s giving us the opposite of a death toll.

So where else might 20 million come from?

It might go as far back as E. D. Morel, who was a clerk for the shipping company Elder Dempster. Thanks to his knowledge of French, he was able to access private accounts relating to the Congo Free State which revealed that the territory was importing all manners of weapons and exporting immensely valuable raw resources. He correctly deduced that the only reasonable explanation was a state-led system of suffering and exploitation.

Morel soon resigned and became an activist against King Leopold’s rule, publishing a stream of papers and criticisms against misrule in the Congo. In a 1904 book, he wrote that “warfare has never ceased for a day in some part or other of the Congo territories. It takes a long time to kill 20 million souls … but there is abundant evidence to show that in parts the native is simply being wiped out.”

Later, he makes an amendment to the population figures to give us a range. In a 1920 book, he wrote that “estimates varied between twenty and thirty millions. No estimate fell below twenty millions. In 1911 an official census was taken. It was not published in Belgium, but was reported in one of the British Consular dispatches. It revealed that only eight and a half million people were left."

Taking his figures together, we arrive at a number of anywhere between 11.5 to 21.5 million deaths. It's worth nothing that this is the same range which appears in the Encyclopaedia Britannica, which might also explain why it became so widespread.

So I think that's where our trail ends. Morel cites the number in the upper end of his range, and while some sources like the Encyclopaedia Britannica might include the entire range, others might have picked the highest number and left it at 20 million.

Sources and further reading

Encyclopaedia Britannica, 15th ed., vol. 3 (1987)

Morel, E. D. King Leopold’s Rule in Africa (1904)

Morel, E. D. The Black Man’s Burden (1920)

Vansina, J. Paths in the Rainforest (1990)

Vanthemsche, G. Belgium and the Congo (2012)