Direct Answer
The Herfindahl-Hirschman Index (HHI) measures industry concentration by summing the squared market shares of every firm in the industry, using whole-number percentages — a firm with 30% market share contributes 30² = 900 to the index. HHI ranges from near zero (many small competitors) to 10,000 (a single-firm monopoly). The U.S. Department of Justice and Federal Trade Commission's Horizontal Merger Guidelines classify a market with HHI below 1,500 as unconcentrated, 1,500–2,500 as moderately concentrated, and above 2,500 as highly concentrated. For investors, a rising HHI often signals a consolidating industry where surviving players gain pricing power, while a falling HHI often signals a fragmenting industry with intensifying competition and margin pressure — a general pattern, not a guarantee.
Key Takeaways
- HHI = sum of squared market shares: Each firm's share, as a whole-number percentage, is squared and the squared values are summed across the industry.
- The index ranges from near 0 to 10,000: Lower values mean more, smaller competitors (closer to perfect competition); higher values mean fewer, larger competitors (closer to monopoly).
- DOJ/FTC thresholds guide merger review: Below 1,500 is unconcentrated, 1,500–2,500 is moderately concentrated, and above 2,500 is highly concentrated under the Horizontal Merger Guidelines.
- Squaring matters: HHI weights larger firms disproportionately more than a simple sum of shares would, because a firm with double the share contributes four times as much to the index.
- Rising HHI often means consolidation: Mergers, acquisitions, or weaker-competitor exits concentrate share among fewer firms, which often — though not always — increases the survivors' pricing power.
- Falling HHI often means fragmentation: New entrants or share losses by incumbents spread the market more evenly, which often increases competitive intensity and pressures margins.
- Market definition drives the result: HHI is highly sensitive to how the relevant market is defined — global, national, or regional; broad category or narrow product line. The same industry can show very different HHI values under different market definitions.
- Not the same as ownership concentration: Market concentration (HHI) measures competitive structure among companies. Institutional ownership concentration measures how concentrated one company's shareholder base is — a different concept entirely.
Core Concepts
How is industry concentration measured?
The simplest way to gauge concentration is a concentration ratio: add up the market shares of the top N firms (commonly CR4, the top four firms, or CR8, the top eight). A CR4 of 80% means the four largest firms control 80% of the market. Concentration ratios are easy to calculate and intuitive, but they ignore the distribution of share both within the top group and among everyone else — a CR4 of 80% split evenly among four firms describes a very different competitive dynamic than a CR4 of 80% where one firm alone holds 70%.
The Herfindahl-Hirschman Index solves this by using every firm's share, not just the top few, and by squaring each share before summing. Squaring means the index is disproportionately sensitive to large firms: a firm with 40% share contributes 1,600 to the index, while a firm with 10% share contributes only 100 — sixteen times less, even though its share is only four times smaller. This weighting reflects the intuition that a single dominant firm changes competitive dynamics far more than the same aggregate share spread across several mid-sized firms.
The HHI formula
HHI = Σ (market sharei)², where each firm's market share is expressed as a whole number out of 100 (so a 30% share is entered as 30, not 0.30), and the sum runs across every firm in the defined market. Using whole-number percentages produces an index that ranges from just above 0 (a large number of firms with negligible individual share, approaching perfect competition) to 10,000 (a single firm with 100% of the market, a pure monopoly).
Analysts and regulators consistently use whole-number percentages rather than decimal shares (which would produce an index from 0 to 1) because the whole-number convention is the one embedded in the DOJ/FTC guidelines and in virtually all published antitrust and industry-research literature — mixing conventions is a common source of confusion when comparing HHI figures across sources.
DOJ/FTC concentration thresholds
The U.S. Department of Justice and Federal Trade Commission's Horizontal Merger Guidelines use HHI to classify markets into three concentration bands: a market with HHI below 1,500 is considered unconcentrated; a market with HHI between 1,500 and 2,500 is considered moderately concentrated; and a market with HHI above 2,500 is considered highly concentrated. These thresholds are used as a screening tool during merger review, not as an automatic legal bar. A proposed merger that would push a market's HHI above 2,500, or that would increase HHI by more than 200 points in an already moderately or highly concentrated market, is more likely to draw closer antitrust scrutiny — but the agencies also weigh entry barriers, buyer power, efficiencies, and other market-specific factors before reaching a conclusion.
These thresholds are useful reference points for investors even outside a merger context, as a rough qualitative gauge of how much room individual firms in an industry have to exercise pricing power without immediately inviting a competitive response or regulatory attention.
What rising or falling HHI implies for investors
A consolidating industry — one where HHI is rising over time, typically through mergers and acquisitions, bankruptcies, or the exit of undercapitalized competitors — often means the surviving players face less competitive pressure on price. Fewer independent decision-makers setting prices generally makes it easier for the remaining firms to sustain price increases without losing meaningful share to a rival, and can also support better capital discipline (less irrational price-based competition for share). This is why industry consolidation is frequently cited as a bullish structural tailwind for the surviving incumbents in sell-side and buy-side research.
A fragmenting industry — one where HHI is falling, typically because of new entrants, technological disruption lowering barriers to entry, or share losses by former leaders — often means intensifying competition and margin pressure, as more independent firms compete for the same customer base and are more willing to compete on price to win or retain share.
Both patterns are general tendencies, not guarantees. A highly concentrated industry can still feature intense non-price competition (on quality, innovation, or service) that limits pricing power despite few competitors. A fragmenting industry can still support healthy margins if overall demand is growing fast enough that competitors don't need to fight over share. HHI describes market structure; it does not, by itself, prove that pricing power exists or that it will be exercised.
Why market definition matters
HHI is only as meaningful as the market definition used to calculate it. The same set of companies can produce very different HHI values depending on whether the relevant market is defined globally, nationally, or regionally, and whether the product category is defined broadly or narrowly. An industry that looks fragmented at the global level (many national champions, none with a dominant global share) can be highly concentrated at the national or regional level (a handful of firms controlling most of a single country's market). Before using or comparing an HHI figure, confirm what market it was calculated over.
Worked Example: Calculating HHI for a Hypothetical 4-Firm Industry
The figures below are a hypothetical illustration, not data for any real industry.
- Define the market: A hypothetical industry has exactly four competing firms, with combined market shares summing to 100%: Firm A holds 40%, Firm B holds 30%, Firm C holds 20%, and Firm D holds 10% (40 + 30 + 20 + 10 = 100%).
- Square each firm's share: Firm A: 40² = 1,600. Firm B: 30² = 900. Firm C: 20² = 400. Firm D: 10² = 100.
- Sum the squared shares: HHI = 1,600 + 900 + 400 + 100 = 3,000.
- Classify the result: An HHI of 3,000 is above the 2,500 threshold, so under the DOJ/FTC framework this hypothetical industry would be classified as highly concentrated.
- Compare to a more fragmented alternative: If the same industry instead had eight firms each holding an equal 12.5% share, HHI would be 8 × (12.5²) = 8 × 156.25 = 1,250 — below the 1,500 threshold, and classified as unconcentrated, despite covering the same total 100% of the market. This illustrates how sensitive HHI is to the number of competitors and the evenness of their shares, not just the aggregate share of the largest players.
- Model a merger: If Firm C and Firm D in the original 4-firm example merged into a single entity with a combined 30% share, the post-merger industry would have three firms: 40%, 30% (original Firm B), and 30% (merged C+D). New HHI = 1,600 + 900 + 900 = 3,400 — an increase of 400 points from the pre-merger 3,000, comfortably above the 200-point change threshold the DOJ/FTC guidelines flag for closer review in an already concentrated market.
HHI Concentration Bands (DOJ/FTC Horizontal Merger Guidelines)
| HHI Range | Classification | General Investment Implication |
|---|---|---|
| Below 1,500 | Unconcentrated | Many competitors; limited individual pricing power; competition-driven margin pressure more likely |
| 1,500 – 2,500 | Moderately concentrated | A meaningful number of firms hold significant share; pricing power varies by industry dynamics |
| Above 2,500 | Highly concentrated | Few firms dominate; surviving players more likely to hold durable pricing power, subject to regulatory and competitive checks |
Common Failure Modes
Using the wrong market definition
Calculating HHI over too broad a market (e.g., "retail" instead of the specific product category and geography actually being competed in) can understate true concentration, while too narrow a definition can overstate it. Always check what market definition underlies a published HHI figure before using it, and be skeptical of comparing HHI figures calculated under different market definitions as if they were directly comparable.
Assuming high HHI automatically means pricing power is being exercised
A highly concentrated industry has the structural conditions that make pricing power easier to sustain, but concentration alone doesn't prove firms are actually raising prices or earning excess margins. Regulatory scrutiny, credible threat of new entry, buyer power (a small number of large, price-sensitive customers), or intense non-price competition can all offset the pricing-power implications of a high HHI. Confirm the concentration signal against actual margin trends and pricing data before concluding pricing power is being realized.
Treating HHI as a static, one-time measurement
Industry concentration changes over time as companies merge, exit, or enter. A snapshot HHI tells you the current structure but not the trend. Tracking HHI over several years (rising, falling, or stable) is generally more useful for investment research than a single point-in-time figure, since the trend indicates whether competitive dynamics are getting better or worse for incumbents.
Confusing market concentration with ownership concentration
"Concentration" is used in stock research to describe two unrelated things, and conflating them leads to confused analysis. Market concentration (HHI) describes how many companies compete for an industry's revenue. Institutional ownership concentration describes how many institutional investors hold a single company's shares. A fragmented, low-HHI industry can still have a stock with highly concentrated institutional ownership, and vice versa — always be explicit about which kind of concentration is being discussed.
Related Swoopr Research
This page covers market concentration — how much of an industry's revenue or output is controlled by a small number of competing companies. This is a different concept from institutional ownership concentration, which measures how much of a single company's shares are held by a small number of institutional investors. See Institutional Ownership Concentration Explained for that separate framework — a reader researching "concentration" in stock analysis should understand these describe two different things before applying either one.
To understand what's actually driving an industry's revenue growth before assessing how concentrated that growth is among competitors, see Industry Growth: Price, Volume, and Mix.
For the full sector and industry research curriculum, start at the Sector & Industry Analysis hub.
FAQ
How do you measure industry concentration?
The most widely used measure is the Herfindahl-Hirschman Index (HHI), calculated by squaring each firm's market share (expressed as a whole number, e.g. 30 for 30%) and summing the squared values across every firm in the industry. A simpler but cruder alternative is a concentration ratio, such as CR4 (the combined market share of the top four firms), which is easier to calculate but ignores the distribution of share among the remaining firms.
How is the Herfindahl-Hirschman Index calculated?
HHI equals the sum of the squared market shares of all firms in the industry, using whole-number percentages. For example, a firm with 30% market share contributes 30 squared, or 900, to the index. HHI ranges from near zero (many firms, each with a tiny share, approaching perfect competition) to 10,000 (a single firm with 100% share, a pure monopoly).
What HHI thresholds does the DOJ and FTC use to classify market concentration?
Under the U.S. Department of Justice and Federal Trade Commission's Horizontal Merger Guidelines, a market with an HHI below 1,500 is generally considered unconcentrated, a market with an HHI between 1,500 and 2,500 is considered moderately concentrated, and a market with an HHI above 2,500 is considered highly concentrated. These thresholds guide merger review rather than acting as an automatic bar — a merger that would push HHI above these levels, or increase it by more than a specified amount, tends to draw closer antitrust scrutiny.
What does a rising HHI mean for investors?
A rising HHI generally signals that an industry is consolidating — market share is becoming more concentrated among fewer firms, often through mergers, acquisitions, or the exit of weaker competitors. This often (though not always) means the surviving players gain pricing power, since fewer competitors makes coordinated or independent price increases easier to sustain without losing customers to a rival. This is a general pattern, not a guarantee — regulatory intervention, new entrants, or substitute products can all offset the pricing-power effect of rising concentration.
Is market concentration the same as institutional ownership concentration?
No, these are different concepts that happen to share the word 'concentration.' Market concentration (measured by HHI) describes how competitive an industry is — how much of the market's revenue or output is controlled by a small number of companies. Institutional ownership concentration describes something entirely different: how much of a single company's shares are held by a small number of institutional investors. A company can operate in a highly fragmented, low-HHI industry while still having highly concentrated institutional ownership of its stock, or vice versa.
Sources
Disclaimer
This article is for educational and informational purposes only and does not constitute personalized investment, financial, or legal advice. The worked example uses hypothetical figures for illustration and does not represent any real industry, company, or merger. Market concentration is one input among many in industry analysis and does not by itself predict company-level pricing power, margins, or stock performance. Trading and investing involve risk, including the possible loss of principal.