A Gini index is even more convenient than a Lorenz curve when the task is to compare income inequality among many countries. The index is calculated as the area between a Lorenz curve and the line of absolute equality, expressed as a percentage of the triangle under the line (see the two shaded areas on Figure 5.2). Thus a Gini index of 0 percent represents perfect equality- the Lorenz curve coincides with the straight line of absolute equality.
A Gini index of 100 implies perfect inequality- the Lorenz curve coincides with the x axis and goes straight upward against the last entry (that is, the richest individual or household; see the thick dotted line on Figure 5.2). In reality, neither perfect equality, nor perfect inequality is possible. Thus Gini indexes are always greater than 0 percent but less than 100 percent (see Figure 5.3).
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From Beyond Economic Growth, World Bank http://www.worldbank.org/depweb/beyond/global/about.html
