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Market concentration
HHI Calculator
Calculates current HHI as the sum of squared market shares and, when two firms are selected, post-merger HHI and delta.
How it works
Enter each undertaking's market share as a percentage. The total should equal 100%. Merger mode combines two shares and compares the before and after values.
Methodology
HHI = Σ(market share²). Results are explained using the initial concentration indicators in the TCA Horizontal Merger Guidelines; they are not findings of legality.
Frequently asked questions
Should I enter 0.35 or 35 for a 35% share?
Enter 35. The calculator uses 35².
Is HHI conclusive on its own?
No. Market definition, barriers to entry, competitive closeness and other conditions also require analysis.