Market Concentration: Concentration Ratios and the HHI Index

Published: May 18, 2026
Ryan O'Connell, CFA, FRM
Article by Ryan O'Connell, CFA, FRM

Market concentration measures how much of an industry’s output or sales is controlled by a small number of firms. For investors analyzing competitive dynamics, understanding concentration is essential. The two most common tools for measuring concentration are concentration ratios and the Herfindahl-Hirschman Index (HHI). This guide explains how to calculate and interpret both measures, their use in merger screening, and their limitations.

What is Market Concentration?

Market concentration refers to the degree to which a small number of firms dominate an industry’s total output, revenue, or market share. A highly concentrated market has most of its sales controlled by just a few large players, while an unconcentrated market has sales spread across many competitors.

Key Concept

Concentration measures market structure, not market power. A market can be highly concentrated yet still competitive if barriers to entry are low and potential competitors can easily enter. Conversely, a moderately concentrated market might exhibit significant pricing power if entry barriers are high.

Understanding concentration helps investors assess:

  • Competitive dynamics — how firms interact and compete
  • Pricing power potential — whether firms can maintain margins
  • Regulatory risk — whether mergers may face antitrust scrutiny
  • Industry stability — how market share shifts over time

Concentration Ratios (CR4 and CR8)

A concentration ratio is the simplest measure of market concentration. It sums the market shares of the largest N firms in an industry.

Concentration Ratio Formula
CRn = s1 + s2 + … + sn
Sum of market shares for the largest n firms

The two most common variants are:

  • CR4 — combined market share of the top 4 firms
  • CR8 — combined market share of the top 8 firms

A CR4 of 80% means the four largest firms control 80% of the market. A CR4 below 40% typically indicates a competitive market, while a CR4 above 60% suggests moderate to high concentration.

Advantages and Disadvantages

Advantage: Concentration ratios are simple to calculate and widely available.
Disadvantage: CR measures are less sensitive to the distribution of shares among top firms. A market with four firms holding 25% each has the same CR4 as a market where one firm has 70% and three others have 10% — yet their competitive dynamics differ significantly.

The HHI Formula

The Herfindahl-Hirschman Index (HHI) addresses the limitations of concentration ratios by squaring each firm’s market share before summing. This weights larger firms more heavily and captures inequality in the size distribution.

HHI Formula
HHI = s12 + s22 + … + sn2
Sum of squared market shares for all firms in the market

When using percentage points (the standard convention for the HHI index), market shares are expressed as whole numbers. A firm with 25% market share enters the formula as 25, and 252 = 625. This produces an HHI scale from 0 to 10,000.

Intuitive Benchmark

An HHI of 1,000 is equivalent to a market with 10 equal-sized firms (each with 10% share: 102 × 10 = 1,000). An HHI of 2,500 is equivalent to 4 equal-sized firms. A monopoly (one firm with 100%) has an HHI of 10,000.

Market share can be measured by revenue, units sold, users, production capacity, or other metrics depending on the industry context.

Interpreting HHI Values

The U.S. Department of Justice (DOJ) and Federal Trade Commission (FTC) use HHI thresholds to classify market concentration. The 2023 Merger Guidelines establish the following bands:

HHI Range Classification Interpretation
HHI < 1,000 Unconcentrated Many competitors, typically competitive markets
1,000 ≤ HHI < 1,800 Moderately concentrated Several significant competitors
HHI > 1,800 Highly concentrated Few dominant firms, potential for market power

The 2023 guidelines lowered the “highly concentrated” threshold from the previous 2010 level of 2,500 to 1,800, reflecting a stricter approach to merger enforcement.

Important Caveat

High concentration does not automatically mean firms have pricing power. A market might be highly concentrated yet competitive if barriers to entry are low and potential competitors can easily enter. Concentration is a structural measure, not a direct indicator of competitive conduct or consumer harm.

HHI in Merger Review

Antitrust regulators use HHI as a screening tool to identify mergers that may warrant closer examination. Under the 2023 DOJ/FTC Merger Guidelines, a merger raises a structural presumption of competitive harm if:

  • The post-merger HHI exceeds 1,800, AND
  • The merger increases HHI by more than 100 points

A separate presumption applies when the merged firm’s market share exceeds 30% and the HHI increase exceeds 100 points.

Merger HHI Change Formula
ΔHHI = 2 × a × b
Where a and b are the merging firms’ market shares in percentage points

For example, if two firms with 15% and 10% market shares merge, the HHI increase is 2 × 15 × 10 = 300 points. This formula makes it easy to assess whether a proposed merger crosses regulatory thresholds.

For Investors

When analyzing potential M&A activity, calculate the post-merger HHI and change in HHI. If both exceed the thresholds above, expect heightened regulatory scrutiny. However, remember that these are screening tools, not definitive legal conclusions — agencies consider many factors beyond HHI.

How to Calculate HHI: Worked Example

Consider a fictional regional building materials market with six firms:

HHI Calculation Example
Firm Market Share Share Squared
Firm A 30% 900
Firm B 25% 625
Firm C 20% 400
Firm D 12% 144
Firm E 8% 64
Firm F 5% 25

CR4 = 30 + 25 + 20 + 12 = 87%

HHI = 900 + 625 + 400 + 144 + 64 + 25 = 2,158

Classification: This market is highly concentrated (HHI > 1,800). The CR4 of 87% confirms that the top four firms dominate.

If Firm A (30%) and Firm D (12%) proposed a merger, the HHI change would be 2 × 30 × 12 = 720 points. The post-merger HHI would be 2,158 + 720 = 2,878, well above both thresholds — this merger would likely face significant regulatory scrutiny.

Try the Market Concentration Calculator

Comparison: HHI vs. Concentration Ratios

Concentration Ratio (CR)

  • Simple to calculate
  • Widely available in industry reports
  • Ignores distribution among top firms
  • Less sensitive to mergers among leaders
  • Best for: quick screening

HHI Index

  • Captures size distribution
  • Sensitive to mergers (via 2ab formula)
  • Used by regulators for merger review
  • Requires all firms’ shares
  • Best for: detailed analysis, M&A

Use concentration ratios when you need a quick gauge of market structure or when detailed share data is unavailable. Use HHI when analyzing potential mergers, assessing regulatory risk, or comparing markets with similar CR values but different size distributions.

Common Mistakes

When working with concentration measures, analysts often make these errors:

1. Equating concentration with market power. A highly concentrated market is not automatically anticompetitive. Low barriers to entry, elastic demand, or the threat of potential competitors can constrain pricing even when HHI is high.

2. Ignoring barriers to entry. A market with high HHI but low entry barriers may be contestable — incumbent firms behave competitively because new entrants can easily enter if prices rise.

3. Using the wrong HHI scale. Academic sources sometimes use a 0-1 scale (decimal shares), while regulators use 0-10,000 (percentage points). Always confirm which scale applies before interpreting thresholds.

4. Aggregating small firms into “Other.” When calculating HHI, if you group many small firms into one “Other” category and treat it as a single firm, the squared term overstates true concentration. Either estimate individual shares or acknowledge the limitation.

5. Ignoring market definition. HHI is only meaningful within a properly defined market. Too narrow a definition inflates concentration; too broad a definition understates it.

Limitations

Structural Measure Limitations

HHI and concentration ratios measure market structure at a point in time. They do not capture competitive conduct, innovation, or how markets evolve.

Key limitations include:

  • Doesn’t capture barriers to entry — the ease of new competitors entering
  • Ignores demand elasticity — how price-sensitive customers are
  • Doesn’t reflect potential competition — firms that could enter if prices rise
  • Market definition sensitivity — results depend heavily on how the market is defined
  • Backward-looking — current shares may not reflect future competitive dynamics
  • No quality or innovation dimension — concentration says nothing about product differentiation or R&D intensity

For a complete picture of competitive dynamics, combine concentration measures with analysis of barriers to entry, customer behavior, and strategic interaction. See our articles on game theory and oligopoly for how firms compete strategically in concentrated markets, monopoly and market power for pricing and welfare implications, and comparing market structures for a broader framework.

Frequently Asked Questions

An HHI of 10,000 indicates a pure monopoly — one firm controls 100% of the market (1002 = 10,000). In practice, markets rarely reach this extreme, but HHI values above 2,500 suggest a market dominated by just a few firms. An HHI of 2,500 is equivalent to four equal-sized firms, each with 25% market share.

CR4 (concentration ratio) simply adds the market shares of the four largest firms. HHI sums the squared market shares of all firms. This difference means HHI is more sensitive to inequality among firms. Two markets with identical CR4 values can have very different HHI values if the distribution of shares differs — for instance, one market with a dominant leader versus another with four similarly-sized firms.

Squaring market shares gives more weight to larger firms, reflecting the intuition that a market with one dominant player is more concentrated than a market with many similarly-sized competitors — even if total market share among top firms is identical. Mathematically, squaring also ensures that a merger between two firms increases HHI by exactly 2 × a × b, where a and b are the merging firms’ shares.

Under the 2023 DOJ/FTC Merger Guidelines, a merger creates a structural presumption of competitive harm if the post-merger HHI exceeds 1,800 and the merger increases HHI by more than 100 points. A separate presumption applies when the merged firm’s market share exceeds 30% with an HHI change over 100. Note that these are screening thresholds, not automatic prohibitions — agencies consider many factors beyond HHI.

No. High concentration indicates market structure, not pricing outcomes. A highly concentrated market may still be competitive if barriers to entry are low (potential entrants discipline pricing), demand is elastic (customers are price-sensitive), or firms compete aggressively despite few players. Conversely, a moderately concentrated market with high barriers to entry may exhibit significant pricing power.

Investors use HHI to assess industry competitive dynamics, potential for margin stability, and regulatory risk from M&A activity. A high HHI may signal potential pricing power and margin durability — but also heightened regulatory scrutiny for acquisitions. A low HHI suggests intense competition but also more opportunity for consolidation. Combine HHI analysis with barriers to entry, customer switching costs, and strategic positioning for a complete competitive picture.

Disclaimer

This article is for educational and informational purposes only and does not constitute investment advice or legal advice. HHI thresholds and regulatory guidelines may change over time. Market concentration measures are analytical tools, not predictions of competitive outcomes or firm profitability. Always conduct your own research and consult qualified professionals before making investment or legal decisions.