Herfindahl–Hirschman Index (HHI) Calculator
Market concentration from a list of market shares — HHI, normalised HHI, the equivalent number of equal firms and the antitrust concentration band.
How it works
HHI = Σ sᵢ² (shares in percent, 0–10,000); normalised HHI = (HHI/10,000 − 1/N) ÷ (1 − 1/N)
The HHI squares every firm’s market share and adds them up, so big players count disproportionately: a monopoly scores 10,000, ten equal firms 1,000. US antitrust guidelines treat below 1,500 as unconcentrated, 1,500–2,500 as moderate and above 2,500 as highly concentrated, and scrutinise mergers that raise HHI by more than 100–200 points. The same index measures portfolio concentration, supplier dependence or the spread of traffic across pages.
Worked example
Shares 35/25/15/10/8/4/3: HHI = 2,224 (moderately concentrated), the equivalent of 4.5 equal firms; merging the top two would add 1,750 points.
Frequently asked questions
Do shares have to sum to 100?
Ideally include every firm. If small firms are missing the HHI is slightly understated — their squared shares are tiny anyway.
What does the merger delta mean?
Merging firms with shares a and b raises HHI by exactly 2ab — the quick screen regulators apply.
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This calculator is for educational purposes. Double-check important results before acting on them.