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For most of history, the world was small
In the year 1 AD there were perhaps 170 million to 400 million people alive. Nobody counted them: scholars work it out from tax lists, harvests and the size of cities, and they disagree. Each grey bar is one year's range of their estimates, never an average. For 1,800 years the line hardly leaves the floor.
scholars’ estimatesthe UN’s count
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The first billion: about 1804
It took all of human history, until about 1804, for the world to hold a billion people at once.
scholars’ estimatesthe UN’s count
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The second: about 1927
The second billion took 123 years, until about 1927. More food, cleaner water and better medicine meant that far more children grew up and had children of their own.
scholars’ estimatesthe UN’s count
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Then a billion every 12 years
The third billion came in 1960, only 33 years later. Each billion from the 5th to the 8th took between 11.5 and 12.4 years. The 8th arrived on 13 Mar 2022, by the UN's count.
scholars’ estimatesthe UN’s countprojection
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Now it is slowing down
Families are smaller almost everywhere, so each new billion takes longer. On the UN's projection the 9th comes in 2037 and the 10th in 2060: 15.2 and 23.5 years. Hatched years are projection, not a count.
scholars’ estimatesthe UN’s countprojection95% range
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A peak, and a range
The UN's middle path peaks at about 10.29 billion around the start of 2084. It is 95% sure the number then lies between 9.51 billion and 11.11 billion (the pale band), and the top of that range is still rising in 2100. The peak is likely; its height and year are not certain.