A common yardstick for inequality is the Gini coefficient or Gini Index, which runs from 0 (everyone has the same income) to 1 (one person has all the income). In most developed countries the Gini coefficient for income ranges between 0.25 and 0.6. Sometimes the index is expressed as range from 0 to 100 (the same thing without the decimal places).
For a succinct description of the index and its calculation, see this post from inequality.org's Salvatore Babones, or the short video below. A more elaborate description of the index, and its mathematical underpinnings, are available from the World Bank.