Free HHI Calculator (Herfindahl-Hirschman Index)

The HHI Index: A Key Metric for Market Concentration

The Herfindahl-Hirschman Index (HHI) is a widely accepted quantitative measure used to evaluate market concentration and competitive dynamics within an industry. A free online HHI calculator allows you to compute this index instantly, making it a valuable tool for antitrust analysis and business strategy. By inputting the market shares of all firms operating in a given market, the calculator delivers an HHI score that reflects the degree of competition or monopoly power present.

Definition and Historical Origin

The HHI was first proposed by Albert Hirschman in 1945 and later popularized by Orris Herfindahl in the 1950s. It is defined as the sum of the squares of the market shares (expressed as percentages) of every firm in the industry. Mathematically, the formula is:

$$HHI = \sum_{i=1}^{n} s_i^2$$

where $s_i$ represents the market share (in percentage) of firm $i$, and $n$ is the total number of firms in the market. The resulting index value ranges from $\frac{10{,}000}{n}$ (indicative of perfect competition) up to $10{,}000$ (representing a pure monopoly).

Step-by-Step Calculation Example

To illustrate the calculation, consider a hypothetical industry with seven competitors. Their respective market shares are shown in the table below.

Enterprise	Market Share (%)
Firm A	35
Firm B	22
Firm C	20
Firm D	10
Firm E	8
Firm F	3
Firm G	2

Applying the HHI formula:

$$HHI = 35^2 + 22^2 + 20^2 + 10^2 + 8^2 + 3^2 + 2^2 = 1225 + 484 + 400 + 100 + 64 + 9 + 4 = 2{,}286$$

An HHI of 2,286 places this industry in the moderately concentrated category (see the interpretation scale below).

Impact of a Merger on HHI

Now suppose Firm B merges with Firm C, creating a combined entity with a 42 % market share. The new market structure comprises six firms, and the recalculated HHI becomes:

$$HHI = 35^2 + 42^2 + 10^2 + 8^2 + 3^2 + 2^2 = 1225 + 1764 + 100 + 64 + 9 + 4 = 3{,}166$$

The index jumps by 880 points, crossing the 2,500 threshold. This change signals a shift to high market concentration, which could raise antitrust flags.

Note: Some conventions express HHI as a decimal fraction. In decimal form, the above values would be 0.2286 and 0.3166, respectively.

Interpreting HHI Scores

Generally, the HHI scale is interpreted as follows:

• HHI < 100 – Highly competitive market (many small firms).
• 100 ≤ HHI ≤ 1,500 – Unconcentrated (no dominant player).
• 1,500 < HHI ≤ 2,500 – Moderately concentrated.
• HHI > 2,500 – Highly concentrated (near‑monopoly conditions).

When evaluating mergers, a post‑merger HHI above 2,500 combined with an increase of more than 100 points is presumed likely to harm competition under U.S. antitrust guidelines.

Practical Use in Antitrust Enforcement

The HHI is a cornerstone of merger review at the U.S. Department of Justice (DOJ) and the Federal Trade Commission (FTC). These agencies use the index to screen proposed transactions and determine whether further investigation is warranted. The European Commission similarly employs HHI thresholds when assessing acquisitions that may affect market competition.

Strengths and Limitations

One major advantage of the HHI is its simplicity: it requires only market‑share data and a straightforward computation, making it accessible to analysts and regulators alike. However, the index captures only static concentration levels and cannot account for all market complexities—such as entry barriers, product differentiation, or pricing dynamics. Therefore, it is best used as a screening tool rather than a definitive verdict on market competitiveness.