Business Tools
Use this HHI calculator to screen market concentration and merger scenarios before doing a deeper competition, pricing, or strategy review.
Live calculator
4 market shares sum to 100.0%.
Based on normalized HHI so incomplete share lists are still comparable 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 |
| Normalized HHI | 2,650 |
| 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 |
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.
Use this for planning and comparison. Contracts, collections, payables, tax timing, payroll, refunds, one-time bills, seasonality, and accounting treatment can change the real business result.
Quick answer
HHI Calculator calculates HHI and merger concentration screen from market shares, merging share A and merging share B. The visible formula is HHI = share1^2 + share2^2 + ... + sharen^2; merger delta HHI = 2 x share A x share B.
Formula
HHI = share1^2 + share2^2 + ... + sharen^2; merger delta HHI = 2 x share A x share BUse market shares from the same relevant market, geography, product scope, and reporting period before comparing HHI values.
How to use
Example
Calculator use
Before relying on it
Details
These notes make the assumptions explicit, especially where the same search query can mean slightly different things.
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.
The delta formula applies to a two-firm merger scenario under the same market definition.
Source notes
Benchmarks
The calculator is a decision aid, not a fixed rule. Use the output to compare scenarios and document your assumptions. Benchmark ranges are broad planning heuristics unless this page names a specific source for the range.
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
The formula, inputs, example, and limitations are shown so the result is checkable, not just a number in a box.
HHI = share1^2 + share2^2 + ... + sharen^2; 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.
June 6, 2026
Toolkit Shelf. HHI Calculator. Last reviewed June 6, 2026. 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.
If entered shares do not total 100%, normalized HHI rescales the entered shares to compare concentration bands. Use raw HHI when the share list covers the full market.
No. HHI is a screening metric. Final assessment depends on market definition, evidence, entry conditions, competition effects, and legal review.
No. Business tools are scenario planners. Contracts, taxes, payment timing, accounting treatment, refunds, and legal requirements can change decisions.
Save the assumptions, compare a conservative scenario, and review the result with actual books, contracts, or an advisor before making a high-stakes decision.
Different tools may use different rounding, assumptions, default rates, methods, formulas, or input timing. Compare the visible method and inputs before relying on the output.