The Gini Index or the Gini coefficient is the most popular tool
  used to calculate inequality by economists.

The 20:20 ratio compares income of the top twenty percent of the
population in a given area to the bottom twenty percent of the group.

See also: https://www.equalitytrust.org.uk/resources2/spirit-level/methods

This tool compares the income of the top ten percent of the population
in a given area to the bottom forty percent.

See also: https://www.cgdev.org/blog/palma-vs-gini-measuring-post-2015-inequality

The Theil index is a statistic used to measure economic inequality
that borrows from information theory.