Market concentration
HHI Calculator
Calculates the current HHI as the sum of squared market shares and, when two firms are selected, the post-merger HHI and the 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 Guidelines on the Assessment of Horizontal Mergers and Acquisitions; they are not findings of legality. The Guidelines subject those indicators to six exceptions: the safe harbour does not apply where a party's pre-merger share is 50% or more.
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.
Can I group small players into a single “Other” row?
Yes for seeing the total share, but no for the HHI. Squaring an aggregated row overstates the HHI, so the tool flags such a row and does not allow it to be selected as a merging party.