In 2005 the World Bank defined the Extreme Poverty Line as $1.25 US a day - that is, someone would be considered to live in extreme poverty if they lived on an amount equivalent to somebody living in the United States, buying United States goods with US$1.25 a day.
The figure is determined by translating the 2005 figure into a local currency figure (using purchasing power parity) and then accounting for inflation since the 2005 date.