How many people died in the 1918 flu pandemic?

How to get close to this number

Work from infection rate and case fatality. The explanation says roughly a third of everyone alive was infected, world population at the time was under two billion, and a case fatality rate of a few percent applied to that gives tens of millions. Population times attack rate times fatality rate is the standard epidemiological decomposition.

The age pattern is the genuinely unusual feature and it is why the death toll was so high. Most respiratory illness kills the very young and the very old; this one hit healthy young adults hardest, probably through an overwhelming immune response. A pandemic that kills the most robust part of a population produces both a higher toll and a very different social impact.

The uncertainty is enormous and honestly stated: estimates range from fifty million to twice that. Record-keeping in 1918 was poor almost everywhere and absent in much of the world, and the war was simultaneously suppressing reporting. The comparison in the explanation — more than both world wars combined — holds even at the low end of the range, which is why it is the framing usually chosen.