Toolkit ShelfUse this HHI calculator to compare standard, share-adjusted, and 0-1 normalized concentration before a deeper competition, pricing, or strategy review.
Live calculator
4 market shares sum to 100.0%.
Based on share-adjusted HHI (0-10,000) so incomplete share lists can be compared with threshold bands.
60.0% combined share; structural presumption triggered.
HHI squares each firm's market share and sums those squared values. Use a consistent market definition before comparing periods or merger scenarios.
| HHI measure | Value |
|---|---|
| Market share entries | 4 |
| Total entered share | 100.0% |
| Raw HHI | 2,650 |
| Share-adjusted HHI | 2,650 |
| Normalized HHI (0-1) | 0.02 |
| Pre-merger concentration | Highly concentrated |
| Merged firm share | 60.0% |
| Change in HHI | 1,750 |
| Post-merger HHI | 4,400 |
| Merger screen | Structural presumption triggered |
Normalized HHI = (HHI on a 0-1 scale - 1/n) / (1 - 1/n), where n is the number of firms. This 0-1 measure adjusts for the equal-share minimum and differs from share-adjusted HHI, which rescales the entered shares to total 100% for concentration-band comparisons.
HHI is a screening metric, not a legal conclusion. Market definition, data source, product scope, geography, entry barriers, buyer power, and agency guidance can change the interpretation.
Planning comparison only. Check estimates, omitted costs, timing, and constraints before making an operating decision.
Formula
HHI = share1^2 + ... + sharen^2; normalized HHI (0-1) = (HHI on 0-1 scale - 1/n) / (1 - 1/n); merger delta HHI = 2 x share A x share BUse one market definition and distinguish 0-1 normalized HHI from share-adjusted HHI used for the 0-10,000 concentration screen.
How to use
Example
Calculator use
Before relying on it
Details
A single firm at 100% share has an HHI of 10,000; many small firms produce a lower score.
The page uses the DOJ/FTC 2023 guideline bands for screening, not as a final legal conclusion.
Uses (HHI - 1/n) / (1 - 1/n) on the 0-1 HHI scale; equal shares produce 0 and a monopoly produces 1.
The delta formula applies to a two-firm merger scenario under the same market definition.
Source notes
Benchmarks
Lower concentration screen; still check market definition and missing share data.
Middle band under the DOJ HHI page; scenario changes can still matter.
A merger delta above 100 points in this band can trigger a structural presumption screen.
Calculator accuracy
HHI = share1^2 + ... + sharen^2; normalized HHI (0-1) = (HHI on 0-1 scale - 1/n) / (1 - 1/n); merger delta HHI = 2 x share A x share B
Market shares, Merging share A, Merging share B
Business results depend on contracts, accounting treatment, taxes, payment timing, refunds, collections, and operating assumptions.
Catalog QA baseline on June 6, 2026. This date marks the catalog-wide automated and editorial QA baseline, not a tool-specific expert review.
Toolkit Shelf. HHI Calculator & Normalized HHI Formula. https://toolkitshelf.com/tools/hhi-calculator-herfindahl-hirschman-index-calculator
FAQ
HHI is the Herfindahl-Hirschman Index, a market concentration measure calculated by squaring each firm's market share and summing the results.
On a 0-1 HHI scale, normalized HHI = (HHI - 1/n) / (1 - 1/n), where n is the number of firms. Equal shares produce 0 and a one-firm monopoly produces 1.
Share-adjusted HHI rescales incomplete entered shares to total 100% and keeps the result on the 0-10,000 scale used for concentration bands. It is different from the 0-1 normalized HHI formula.
No. HHI is a screening metric. Final assessment depends on market definition, evidence, entry conditions, competition effects, and legal review.
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