Concentration Ratio o Concentration ratio is the percentage of an industry's total output accounted for by the largest firms A typical measure is the four-firm concentration ratio. which is the fraction of output accounted for by the four largest firmsConcentration Ratio  Concentration ratio is the percentage of an industry’s total output accounted for by the largest firms.  A typical measure is the four-firm concentration ratio, which is the fraction of output accounted for by the four largest firms