Herfindahl-Hirschman Index o HHI is calculated by squaring the market share of each firm competing in a market and then summing the resulting numbers. H=∑S2 o HHI ranges from a minimum of close to 0 to a maximum of 10.000Herfindahl-Hirschman Index  HHI is calculated by squaring the market share of each firm competing in a market and then summing the resulting numbers.  i HHI Si ＝ 2  HHI ranges from a minimum of close to 0 to a maximum of 10,000