<P> It is estimated that the world population reached one billion for the first time in 1804 . It was another 123 years before it reached two billion in 1927, but it took only 33 years to reach three billion in 1960 . Thereafter, the global population reached four billion in 1974, five billion in 1987, six billion in 1999 and, according to the United States Census Bureau, seven billion in March 2012 . The United Nations, however, estimated that the world population reached seven billion in October 2011 . </P> <P> According to current projections, the global population will reach eight billion by 2024, and will likely reach around nine billion by 2042 . Alternative scenarios for 2050 range from a low of 7.4 billion to a high of more than 10.6 billion . Projected figures vary depending on underlying statistical assumptions and the variables used in projection calculations, especially the fertility variable . Long - range predictions to 2150 range from a population decline to 3.2 billion in the "low scenario", to "high scenarios" of 24.8 billion . One extreme scenario predicted a massive increase to 256 billion by 2150, assuming the global fertility rate remained at its 1995 level of 3.04 children per woman; however, by 2010 the global fertility rate had declined to 2.52 . </P> <P> There is no estimation for the exact day or month the world's population surpassed one or two billion . The points at which it reached three and four billion were not officially noted, but the International Database of the United States Census Bureau placed them in July 1959 and April 1974 respectively . The United Nations did determine, and commemorate, the "Day of 5 Billion" on July 11, 1987, and the "Day of 6 Billion" on October 12, 1999 . The Population Division of the United Nations declared the "Day of 7 Billion" to be October 31, 2011 . </P> <Table> <Tr> <Td> 80 + 77.5 + 75 + 72.5 + 70 + 67.5 + 65 + </Td> <Td> 60 + 55 + 50 + 45 + 40 + less than 40 </Td> </Tr> </Table>

When was the world's population half of what it is now