wikipedia

Milestones

The following shows estimates of when each billion milestone was or will be met:

Population	1 billion	2 billion	3 billion	4 billion	5 billion	6 billion	7 billion	8 billion	9 billion
Year	1804	1927	1960	1974	1987	1999	2011	2024	2042
Years until next billion	123	34	13	13	12	12	13	18

These numbers show that the world's population has tripled in 72 years, and doubled in 38 years up to the year 1999. Including some more estimates, the world population has been doubled or will double in the following years (with two different starting points). Note how, during the 2nd millennium, each doubling has taken roughly half as long as the previous doubling.

Population	250 million		500 million		1 billion		2 billion		4 billion		8 billion
Year	AD 950		1600		1804		1927		1974		2024
Years until next doubling		650		204		123		47		50
Population		375 million		750 million		1.5 billion		3 billion		6 billion
Year		1420		1720		1875		1961		1999
Years until next doubling			300		155		86		38