HHI Calculator (Herfindahl-Hirschman Index Calculator)

Written by Great Medico

Updated September 25, 2026 •2 min read

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HHI Calculator (Herfindahl-Hirschman Index Calculator) turns a familiar business question into a compact set of numbers, so you can see the relationship without building a separate spreadsheet.

What this calculator does

HHI Calculator (Herfindahl-Hirschman Index Calculator) measures market concentration by squaring each entered firm market share and summing those squared shares into a Herfindahl-Hirschman Index. That makes the output useful for the specific relationship being measured here, while keeping any unentered business or investment assumptions outside the calculation.

How to use it

Enter Firm 1, Firm 2, Firm 3, Firm 4, Firm 5, Firm 6, and the remaining displayed fields. Use figures from the same reporting period and the same accounting, workforce, inventory, or campaign definition wherever possible. Before using the result in a decision, recheck unusually large or negative values against the source data rather than assuming the calculator is correcting an inconsistent input.

How the calculation works

HHI = the sum of the squared market shares entered as percentages. A market with shares of 30%, 25%, 20%, 15%, and 10% has HHI = 30²+25²+20²+15²+10². The numeric index ranges toward 10,000 as concentration approaches a single-firm market.

Example

The default shares 30%, 25%, 20%, 15%, 10% total 100%. Squaring and summing them gives an HHI of 2,250.

How to interpret the result

The HHI number is a concentration measure, not a legal conclusion. Current U.S. DOJ/FTC merger guidance describes markets above 1,800 HHI as highly concentrated and looks at the change in HHI as well; this calculator’s legacy status text may use older concentration bands, so rely on the numeric HHI and the current jurisdiction-specific rule when legal interpretation matters.

Limitations and notes

Market shares must use a coherent market definition and should total 100%. More importantly, legal concentration thresholds change over time and differ by jurisdiction. The arithmetic HHI is durable; any “unconcentrated/moderate/high” label should be checked against current enforcement guidance before it is used in antitrust analysis.