Reaching each new billion took less and less time — until recently. Here is the timeline: 1 billion in 1804 (123 years after the first reliable estimates), 2 billion in 1927 (123 years), 3 billion in 1960 (33 years), 4 billion in 1974 (14 years), 5 billion in 1987 (13 years), 6 billion in 1999 (12 years), 7 billion in 2011 (12 years), 8 billion in 2022 (11 years).

Notice the acceleration through the middle of the 20th century, then the plateau: each billion now takes roughly the same time, about 11–12 years. This reflects the global fertility decline that began in the 1960s.

Before 1804, estimates are rough. The world reached 100 million around 1000 BCE, 250 million at the time of the Roman Empire, and about 500 million around 1500. The Black Death in the 14th century may have killed a third of Eurasia's population.

The next milestones will arrive more slowly. The UN projects 9 billion around 2037 and 10 billion around 2058. After that, growth stalls: the peak of about 10.4 billion is expected in the 2080s.

These milestones are estimates, not exact moments. No one was counting in 1804, and even today the count is a statistical reconstruction. The milestone is a symbol — a way to grasp the scale of human growth.