Herfindahl-Hirschman Index Calculator

Enter up to 10 firms with their company name and market share (%) to compute the Herfindahl-Hirschman Index (HHI). You'll get the HHI score, a market concentration classification (Unconcentrated, Moderately Concentrated, or Highly Concentrated), plus a breakdown chart showing each firm's squared market share contribution.

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Enter the percentage market share for this firm (0–100).

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Results

Herfindahl-Hirschman Index (HHI)

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Market Concentration

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Number of Firms

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Total Market Share

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Equivalent Competitors (1/HHI × 10,000)

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Squared Market Share Contribution by Firm

Results Table

Frequently Asked Questions

What is the Herfindahl-Hirschman Index (HHI)?

The HHI is a measure of market concentration calculated by summing the squares of each firm's percentage market share. It ranges from near 0 (perfect competition) to 10,000 (pure monopoly). Regulators use it to assess how competitive a market is and to evaluate the impact of mergers and acquisitions.

How is the HHI calculated?

Square each firm's market share percentage and add all the squared values together. For example, a market with two firms holding 70% and 30% has an HHI of 70² + 30² = 4,900 + 900 = 5,800. Our calculator does this automatically as you enter each firm's share.

What HHI score indicates a highly concentrated market?

According to the DOJ-FTC 2023 Horizontal Merger Guidelines, an HHI above 1,800 indicates a highly concentrated market. An HHI between 1,000 and 1,800 signals moderate concentration, and below 1,000 is considered unconcentrated (competitive).

How do regulators use the HHI for merger reviews?

The U.S. Department of Justice and the Federal Trade Commission use the HHI to screen proposed mergers. A merger that raises the HHI by more than 100 points in a highly concentrated market (HHI > 1,800) — or results in a merged firm with more than 30% share — is presumed to substantially lessen competition.

What does a monopoly look like on the HHI scale?

A pure monopoly, where a single firm holds 100% of the market, produces an HHI of 10,000 (100² = 10,000). As more equally sized competitors enter the market, the HHI falls. A market with 10 equal firms each holding 10% would have an HHI of 1,000.

What is the 'Equivalent Competitors' metric?

Equivalent Competitors (also called the Numbers Equivalent) is calculated as 10,000 divided by the HHI. It tells you how many equally-sized firms would produce the same HHI. For example, an HHI of 2,500 is equivalent to a market with 4 firms of equal size.

Do all firms' market shares need to add up to 100%?

Ideally yes — the HHI is most meaningful when the market shares of all firms sum to 100%. If your shares total less than 100%, it may indicate smaller firms or a fringe that you haven't listed individually. This calculator shows your total share so you can verify coverage.

What is the difference between a pre-merger and post-merger HHI?

Regulators calculate the HHI both before and after a proposed merger to measure the change (ΔHHI). A post-merger HHI above 1,800 combined with a ΔHHI above 100 points triggers regulatory scrutiny. You can simulate this by combining two firms' shares into one and recalculating with this tool.

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