How many people lived in the world in 1900?

How to get close to this number

Work back from today. World population is now around eight billion, and the explanation gives the ratio: roughly a fifth of today's figure. Dividing the present by five is far easier than recalling a historical total, and ratios between a known present and a known past are the most reliable way to handle historical demography.

The framing in the explanation is the one worth carrying: it took all of human history to reach that point, and then the figure quintupled in a single century. Population growth was effectively flat for millennia and then vertical for a hundred years, which is why every intuition built on historical experience fails at describing the twentieth century.

The cause is the counterintuitive part: the explosion came from falling child mortality rather than rising birth rates. People did not start having more children; their children stopped dying. Population growth is a mortality story far more often than a fertility story, and getting the direction of causation right changes what you conclude about the future.