hhi index calculator

What Is the HHI Index?

The Herfindahl-Hirschman Index (HHI) is one of the most widely used measures of market concentration. It helps you answer a simple question: Is this market competitive, or dominated by a few firms?

Regulators, economists, strategy teams, and investors all use HHI to evaluate industry structure. A higher score means market power is concentrated in fewer hands; a lower score suggests a more competitive landscape with many similarly sized players.

HHI Formula

The HHI is calculated by squaring each firm's market share and summing the squares:

HHI = s₁² + s₂² + ... + s_n²

• If shares are percentages (like 40%, 30%, 20%, 10%), HHI ranges from 0 to 10,000.
• If shares are decimals (0 to 1), HHI ranges from 0 to 1.
• This calculator reports both for convenience.

Quick Example

Suppose a market has four firms with shares 40%, 30%, 20%, and 10%. HHI = 40² + 30² + 20² + 10² = 1600 + 900 + 400 + 100 = 3000. That indicates a highly concentrated market.

How to Use This Calculator

1. Enter firm values in the input field (comma or space separated).
2. Leave the checkbox on if those values are already percentages.
3. Turn the checkbox off if values are raw sizes (the calculator converts them to shares).
4. Optionally enter two firm indices to simulate a merger.
5. Click Calculate HHI to get results and interpretation.

How to Interpret HHI Results

A common interpretation framework (often used in antitrust analysis) is:

HHI (0–10,000 scale)	Market Structure	Typical Interpretation
Below 1,500	Unconcentrated	Generally competitive market conditions
1,500 to 2,500	Moderately concentrated	Some concentration, but not extreme
Above 2,500	Highly concentrated	A few firms may hold significant market power

Merger Analysis and Delta HHI

In merger reviews, analysts look at both the post-merger HHI and the change in HHI (ΔHHI). Even a moderate shift can matter in an already concentrated market.

• Post-merger HHI high + large ΔHHI can signal stronger antitrust concern.
• Low post-merger HHI + small ΔHHI usually indicates lower structural risk.
• HHI is only one test; regulators also evaluate entry barriers, customer choice, and innovation impacts.

When HHI Is Useful Beyond Regulation

1) Corporate Strategy

A firm can use HHI to estimate competitive intensity before entering a market. Lower concentration often means more rivals and pricing pressure. Higher concentration may imply stronger incumbents, but potentially clearer niches.

2) Investing and Industry Research

Analysts can compare HHI across industries to understand structural differences. For example, telecom or airlines may trend higher than fragmented service categories.

3) Portfolio and Revenue Diversification

The same math can measure concentration in customer mix, product mix, or supplier dependence. A very high internal HHI can be a warning sign for concentration risk.

Common Mistakes to Avoid

• Mixing percentages and raw values in the same input set.
• Forgetting to use consistent market boundaries (geography, product definition, period).
• Treating HHI as a final legal verdict rather than a screening indicator.
• Ignoring fast-changing markets where current shares may quickly become outdated.

FAQ

Does HHI require percentages that sum exactly to 100?

In principle, yes. This calculator can auto-normalize values if totals are slightly off. That helps with real-world data that may contain rounding error.

Can I use sales dollars instead of percentages?

Yes. Uncheck “Treat values as market shares (%)”. The tool will convert raw values to share percentages automatically.

Is a high HHI always bad?

Not necessarily. Some markets are naturally concentrated due to scale economics or infrastructure needs. A high HHI is a signal to investigate more deeply, not a standalone conclusion.

Bottom Line

HHI is a simple but powerful concentration metric. With one input list, you can quickly quantify market structure, compare industries, and evaluate potential merger impact. Use it as a first-pass analytical tool, then pair it with qualitative market context for better decisions.