Monopoly MOAT Research Desk FREE TOOL

Market concentration calculator

Is this market dominated by a few companies?

Enter each company’s market share. We’ll show how concentrated the market is in plain English.

Start with the example

No signup
Works in your browser
Uses current US thresholds

Educational tool · reviewed August 29, 2026 · not a legal conclusion

START WITH THIS EXAMPLE

Check the example, then make it yours.

Example: a fictional neighborhood coffee market. You do not need to know the HHI formula to use this calculator.

1. Review or replace the example shares.
2. Make sure the shares add up to 100%.
3. See what the result means.

Company market shares

Use the same product, place, year, and measure for every company. Replace the fictional coffee example when you are ready.

Review or replace the example

Enter every separately identified company you can support.

Company or participant Share of market%

Reach 100%

The calculator needs the whole market to give you a meaningful label.

Market shares entered100.00%

Shares total 100%. You’re ready to check the market.

Where do the shares come from—and what if I don’t know them all?

Use company filings, regulator reports, trade-association data, or a clearly documented estimate. Keep the product, place, measurement basis, and date consistent.

If you cannot account for the full market, do not force a result. Find a more complete source or document the missing share outside this calculator. Combining unknown competitors into one “Other” row usually overstates concentration because HHI squares that combined share.

Optional: see what happens if two companies combine

First company Second company

The calculator treats these two as one company and leaves every other share unchanged.

Read the result

The plain-English answer comes first. Numbers and method stay optional.

Company names, shares, selections, and results stay in this browser and are never sent to Monopoly MOAT or PostHog.

How HHI works and what the thresholds mean

THE MATH

Square each share. Then add the answers.

HHI is the sum of each company’s squared market share. Squaring gives larger companies more weight. For the DOJ’s 30%, 30%, 20%, 20% example: 30² + 30² + 20² + 20² = 2,600.

HHI= Σ market share²

two-company change= 2 × share A × share B

Under 1,000: unconcentrated.

1,000 through 1,800: moderately concentrated. Exactly 1,800 does not meet a strict “above 1,800” screen.

Above 1,800: highly concentrated.

Under the 2023 Merger Guidelines, one numerical screen asks whether the post-merger HHI is above 1,800 and the increase is above 100. Another asks whether the merged company is above 30% and the increase is above 100.

This is a screening tool, not a legal verdict. The market definition and underlying evidence still matter.

WHY THE EXAMPLE IS FICTIONAL

Real market shares need real sources.

A real market needs a defensible product and geography, dated shares covering the whole market, one measurement basis, and publication rights. The fictional coffee-shop example teaches the tool without pretending that an unsourced market is real.

COMMON QUESTIONS

Questions readers usually ask.

What is the Herfindahl-Hirschman Index?

HHI is a market-concentration measure calculated by squaring every participant's percentage market share and adding the results. It ranges from close to zero in a market with many tiny participants to 10,000 for a single-firm market.

What HHI is considered highly concentrated in the United States?

The 2023 US Merger Guidelines describe a market with HHI greater than 1,800 as highly concentrated and a change greater than 100 points as significant. Those numbers are screening indicators, not a finding that conduct or a transaction is unlawful.

Why must the market shares add to 100%?

A partial market can materially understate HHI. This calculator requires shares to total 100% within a small rounding tolerance and explains exactly how much is missing or over; it never silently normalizes missing competitors or labels an incomplete market.

Should I enter all small competitors as one Other row?

No. Squaring one combined Other share usually overstates the contribution of those competitors. Enter separately identified firms separately. If the tail cannot be resolved, label the market incomplete rather than inventing a split.

Does a high HHI prove a monopoly or an illegal merger?

No. The relevant product and geographic market, the reliability and timing of shares, direct competitive evidence, entry, efficiencies, and other facts matter. This tool reproduces a structural screen; it does not provide a legal conclusion or investment recommendation.

SOURCES AND DATES

Check the sources behind the result.

Each figure keeps its source date. Unknowns and derived calculations are labelled rather than filled in.

01Herfindahl-Hirschman Index explainerUS Department of Justice · updated Jan 17, 2024
022023 Merger Guidelines, Guideline 1US DOJ and Federal Trade Commission · Dec 18, 2023
03Market definitionUS Department of Justice · 2023 Merger Guidelines
04Market sharesUS Department of Justice · 2023 Merger Guidelines