When Does Market Concentration Become Market Power?

A business can lead a market without being able to raise prices at will. A local market can look concentrated while a national market looks crowded, or the reverse. For a family office evaluating a direct investment or a proposed acquisition, a concentration number is a signal to investigate, not a verdict on pricing power, legality or future returns.

The work has three stages. Define what customers can actually buy instead, and where. Measure who sells into that market. Then test whether customers, rivals and potential entrants constrain the company. A four-firm concentration ratio (CR4) or Herfindahl–Hirschman Index (HHI) answers the middle question. It cannot finish the third.

01Concentration counts sellers; market power tests constraints

Market concentration describes how sales or another appropriate measure are distributed among firms in a defined market. If the largest firms hold most of the shares, concentration is high. Market power concerns whether a firm can profitably influence price, output, quality, wages or other competitive terms because rivals and customers cannot discipline it sufficiently. The two ideas can be related, but they are not the same measurement.

The Richmond Fed's economic brief explains why: market share is relatively easy to observe, while markups and the degree of power are much harder to measure. It also reports that national and local concentration trends can diverge. Its empirical result is about the markets and historical data studied, not proof that any present target company lacks or holds power.

For an investor, “dominant company” is therefore a hypothesis. Could a customer switch to another supplier next quarter? Could an adjacent product substitute? Could a new entrant reach the market at viable cost? Does the company win sales through lower costs or better quality rather than the ability to raise price? The answers determine what a share number means for underwriting.

02Choose the denominator before using CR4 or HHI

Suppose the family office is considering an investment in firm C, which serves a fictional regional industrial-service market. C's plan includes buying firm D. Before accepting any reported share, including market leader A's 40%, document which service, which buyers, which geography, which year, and whether the denominator is revenue, units, capacity or another measure. Keep all firms on the same basis and time period. A share of one niche service cannot be combined with rivals' shares of a broader bundle without rebuilding the data.

Customers' real alternatives may cross the selected boundary. A national share may hide a highly constrained local market; a local share may omit a digital or neighboring-region supplier that buyers actually use. The Richmond Fed's national/local analysis is a warning against choosing geography for convenience. The relevant market remains a case-specific economic and legal question.

Now assume six firms have shares of 40%, 25%, 15%, 10%, 5% and 5% on one chosen sales denominator and date in the same fictional product and local geography. These are illustrative inputs, not verified market data. Their shares total 100%. The four largest hold 40+25+15+10 = 90%, so CR4 is 90%.

The US Department of Justice defines HHI as the sum of each firm's squared percentage share. Here it is 40²+25²+15²+10²+5²+5² = 2,600. HHI gives a 40% firm more weight than four separate 10% firms; CR4 does not show the distribution within its top-four total.

Neither number tells us whether C or market leader A can raise prices. If buyers can switch among several firms or import an equivalent service, even A's high local share may overstate its ability to raise prices. If capacity is scarce and customer switching is costly, shares alone may miss the weak competitive pressure. Those are matters for evidence, not assumptions embedded in HHI.

03Test a proposed acquisition without calling the index a verdict

Now test C's proposed purchase of D. C has 15% and D has 10%. Assume for the illustration that all sales remain in the defined market, the other shares do not change, and the two firms become one competitor. The combined firm would hold 25%, leaving shares of 40%, 25%, 25%, 5% and 5%. The top four now hold 95%, up from 90%.

The combined firm's HHI contribution changes from 15²+10² = 325 to 25² = 625. The post-transaction HHI is 2,900, up 300 points. The same HHI increase can be checked as 2×15×10 = 300.

Under the DOJ and FTC's 2023 Merger Guidelines, a merger in a relevant market with post-merger HHI above 1,800 and an increase above 100 points meets the Guidelines' rebuttable structural presumption of illegality. This fictional calculation crosses those stated thresholds if the market definition and shares hold up. The presumption is a merger-screening and legal-analysis issue, not a finding that either current firm already has market power or that the acquisition will certainly be blocked.

Before the family office values its proposed stake in C on an acquisition-dependent basis, ask antitrust counsel to test the real product/geographic market and the competitive relationship between C and D. The office should not model a synergy that depends on price increases or reduced service until the economic and legal case is examined. It should preserve the underlying sales data, competitor list and customer evidence for the advisers; a polished 2,900 score without source lineage is not diligence.

04Look for pricing power in customer and rival evidence

High concentration may coexist with strong constraints, and a lower concentration figure may miss a narrow group of locked-in customers. To test the investment thesis, work from transactions and behavior rather than a slogan about a moat.

The family office should record what was observed, for which customers, over which dates, and which explanation remains viable. A price rise with no lost customers is informative only if the office knows whether contracts delayed switching, rivals were capacity-constrained, or quality improved. Evidence from one region should not be silently applied to every region.

For C, use a dated customer cohort and compare realized net price, volume or churn, and unit margin before and after a price change. Record the contract and transaction source, then test whether input costs, improved quality or rival capacity explain the result. No such result has been supplied in this fictional case, so the base valuation carries no pricing-power premium. If customers switch or margin gains vanish after costs, keep the premium at zero. If repeated, source-checked cohorts show sustained net-price and margin gains with limited switching and weak rival response, the committee may test a sensitivity; that evidence still does not guarantee future returns.

For a direct investment, concentration can be a risk as well as an advantage. A target dependent on a concentrated set of customers or suppliers may have less bargaining power even while its own product market is concentrated. Keep seller concentration, customer concentration and the family's portfolio exposure in separate columns. One HHI cannot stand for all three.

05Turn the finding into an investment-committee decision

The committee should decide the next diligence action, not vote “market power: yes” from a high ratio. A compact record can separate the observed number from the inference and from the office's investment decision.

If the evidence later shows a defensible market, sustained price or margin behavior and weak competitive constraints, the office may have a stronger investment thesis. It still must value execution, regulation and customer response separately. If the evidence instead shows easy switching or rapid entry, high concentration should not be translated into a durable moat.

Start with the target's last two years of customer-level price and win/loss records, then rebuild the market-share denominator around the buyers' actual alternatives. The concentration calculation is quick; proving what it means is the investment work.