Data is calculated by OurWorldinData as the net change in population from one year to the next e.g. figures for 1900 represent the net change in population between 1900 and 1901.
These calculations are based on the 'OurWorldinData'-series derived from various sources. The data for the period before 1900 are taken from the History Database of the Global Environment (HYDE). The History Database of the Global Environment (HYDE) collected the data by earlier publications. The data for the World Population between 1900 and 1940 is taken from the UN publication 'The World at Six Billion'. The annual data for the World Population between 1950 and 2015 is taken from the United Nations, Department of Economic and Social Affairs, Population Division (2017). World Population Prospects: The 2017 Revision, DVD Edition. Available at: https://esa.un.org/unpd/wpp/Download/Standard/Population/
Data from 2016 onwards is based on the above UN source's 'medium variant' projection to the year 2100.
The HYDE Database is available at: http://themasites.pbl.nl/tridion/en/themasites/hyde/basicdrivingfactors/population/index-2.html
The full 'OurWorldinData' series is available to download at: https://ourworldindata.org/wp-content/uploads/2013/05/WorldPopulationAnnual12000years_interpolated_HYDEandUNto2015.csv