Herfindahl-Hirschman Index Calculator
Calculate the Herfindahl-Hirschman Index (HHI) from market shares to measure market concentration.
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How to use this tool
- List each competitor's market share as a percentage, separated by commas (they should sum to roughly 100).
- Read the HHI value and its concentration classification.
The Herfindahl-Hirschman Index (HHI) measures how concentrated a market is by summing the squares of each competitor's market share. Regulators like the U.S. DOJ and FTC use HHI thresholds to flag mergers that could reduce competition — enter each firm's share to calculate it.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Frequently asked questions
- How is HHI calculated?
- Square each firm's market share (as a percentage, not a decimal) and add them up. Four firms with 25% shares each give HHI = 25² × 4 = 2500.
- What HHI counts as 'highly concentrated'?
- Under U.S. DOJ/FTC merger guidelines, HHI below 1,500 is unconcentrated, 1,500–2,500 is moderately concentrated, and above 2,500 is highly concentrated — mergers pushing HHI above these thresholds draw extra scrutiny.
- What's the maximum possible HHI?
- 10,000, representing a pure monopoly with a single firm holding 100% of the market (100² = 10,000).