HHI Merger Thresholds: Apply the 2023 Structural Screen
HHI merger thresholds are part of a dated analytical framework, not an automatic verdict. Calculate concentration in the relevant market, compare the post-merger level and change with the stated screen, and explain what the result supports. The 2023 U.S. guidelines also describe a separate merged-share branch.
An HHI number is a screen within a broader competitive analysis.
Apply Two Dated Screen Examples
Under the 2023 U.S. Merger Guidelines screen, a merger raises HHI from 1,600 to 1,920. The increase is 320 points. How should the screen be applied?
- Neither threshold is met
- Only the HHI-change threshold is met
- The post-merger level exceeds 1,800 and the increase exceeds 100, so the stated structural screen is met
- The merger is automatically unlawful
- HHI alone proves consumer harm
The post-merger level exceeds 1,800 and the increase exceeds 100, so the stated structural screen is met This calculation applies the dated 2023 screen. It identifies a structural presumption, not an automatic final judgment. Analysts must also use the guidance and evidence current at the time of review.
Using that same dated 2023 screen, a merger raises HHI from 2,500 to 2,580. Which statement is accurate?
- Both stated thresholds are met
- The level exceeds 1,800, but the 80-point increase does not exceed 100
- The market is unconcentrated because the change is small
- The merger is automatically lawful
- The HHI must be divided by the number of firms
The level exceeds 1,800, but the 80-point increase does not exceed 100 The example fails the stated change condition even though the level is high. Thresholds organize an inquiry. They do not replace market definition, competitive-effects evidence, or a check for later guidance.
Video Lesson: Calculating Market Concentration
A focused video lesson from Economics in Many Lessons.
Read the Stated HHI Conditions
The 2023 U.S. Merger Guidelines describe a market with HHI above 1,800 as highly concentrated and treat an increase above 100 points as significant in the structural presumption described there. For economics exam purposes, however, the durable skill is calculating HHI and interpreting a rise as increased concentration. A question should supply any enforcement threshold it expects you to apply because policy guidance can change.
Check the Market Behind the Number
Even a correct HHI can mislead if the market is defined too narrowly or too broadly. A merger of two local sellers looks more concentrated when the relevant market is one town than when customers readily buy from a national online market. Evidence about entry, substitution, capacity, coordination, and buyer power completes the analysis. Concentration raises a question. It does not by itself prove monopoly conduct.
Calculate the Merger’s HHI Change
Two firms with shares 15 and 10 merge. The HHI increase attributable to combining them is 2(15)(10)=300, the same result obtained by recomputing the squared shares.
Distinguish Share from Market Power
Ask whether customers have alternatives, rivals can enter, and a seller can profitably sustain price above the competitive level. Share is evidence within that analysis.
Keep Guidance and Durable Arithmetic Separate
Concentration thresholds in agency guidance are analytical screens, not timeless economic constants. Agencies revise guidelines as economic evidence, law, and enforcement priorities change. A test of durable economics should either supply the threshold it wants applied or ask for a qualitative conclusion such as “the merger raises HHI and concentration.” Memorizing one year’s number is less valuable than understanding the index and its limitations.
Follow the Order of Analysis
Even when a threshold is supplied, the order of analysis matters. Define the relevant product and geographic market, calculate premerger HHI, calculate the postmerger level and change, and then consider competitive conditions. A structural presumption based on concentration can focus investigation. It does not replace evidence about substitution, entry, capacity, coordination, or efficiencies.
Define the Product and Geographic Market
Two clinics have shares of 30 and 20 percent among providers within a small county. If patients readily travel to neighboring counties or use telehealth, those shares may overstate constraint lost by a merger. If emergency patients cannot travel and insurers need both clinics, the local definition may be economically meaningful. The correct boundary follows customer substitution, not political lines alone.
Evaluate Feasible Entry and Expansion
Entry must be timely, likely, and sufficient to discipline a price increase. The mere possibility that someone could open a business is not enough if licenses take years, customers face switching costs, essential inputs are locked up, or minimum efficient scale is large. Conversely, firms currently outside the market may provide strong potential competition if they can redirect capacity quickly.
Consider Changing Competition and Buyer Power
Market shares can also change rapidly in innovative industries. A snapshot may miss platform effects, data advantages, ecosystem lock-in, or a disruptive entrant. Strong buyers can sometimes resist price increases, but buyer power protecting a few large purchasers may not protect smaller customers. Imported products matter only to the extent they are credible substitutes and supply can expand.
Connect Efficiency Claims to Outcomes
Efficiency claims require discipline. Lower marginal cost can lead to lower prices or improved quality, especially if competition remains, but a firm with greater power may retain some gains. Fixed-cost savings affect profitability yet, holding continued operation and other conditions fixed, do not automatically change the profit-maximizing marginal decision. An exam distractor may cite “economies” without connecting them to consumer outcomes.
State the Supported Conclusion
Keep legal and economic language separate. Higher concentration means the market structure became more concentrated. It does not logically prove an antitrust violation, collusion, or consumer harm. Likewise, a threshold below a presumption does not prove the merger harmless. Use the wording supplied in the stem and do not claim more certainty than the evidence supports.
Read the Official Framework and Its Limits
The Justice Department’s HHI definition sums squared percentage-point market shares. Use the same relevant market and share measure before and after the hypothetical merger. Decimal shares produce a differently scaled number; do not compare it directly with thresholds stated on the 0–10,000 scale.
Guideline 1 of the 2023 U.S. Merger Guidelines states two structural branches: post-merger HHI greater than 1,800 together with an increase greater than 100, or merged-firm share greater than 30% together with an increase greater than 100. The inequalities are strict. A value exactly at a boundary does not exceed it. These are the conditions of that named framework, not a universal rule for every jurisdiction or industry.
The guidelines’ overview describes other ways mergers can raise competitive concerns. Guideline 1 also explains that its presumption can be rebutted or disproved. Failing the HHI branch does not establish that a transaction is harmless or lawful. Check the applicable framework and competitive evidence rather than using a threshold as a substitute for the inquiry.
Recompute a Complete Hypothetical Market
Seven firms have percentage shares of 18, 12, 14, 14, 14, 14 and 14. The 18% and 12% firms merge, and all other shares remain unchanged in the same market. Calculate the pre-merger HHI, post-merger HHI and change. Then apply only the stated 2023 Guideline 1 structural conditions.
Check your reasoning
The shares sum to 100%. Before the merger, HHI = 18² + 12² + 5 × 14² = 324 + 144 + 980 = 1,448. Afterward, the merged share is 30%, so HHI = 30² + 5 × 14² = 900 + 980 = 1,880. The change is 432, matching 2 × 18 × 12. The HHI branch is met because 1,880 exceeds 1,800 and 432 exceeds 100. The separate share branch is not met because 30% does not exceed 30%. Meeting the first branch identifies its rebuttable presumption; it does not settle the final legal outcome.
Try Three HHI-Screen Questions
Choose one answer, then check your reasoning. These are short learning checks, not an exam score. If the controls are unavailable, the explanations remain readable below each question.
Answer B. The change is 1,900 − 1,750 = 150. The two strict inequalities are satisfied, so the stated branch produces a rebuttable structural presumption. It is not an automatic final judgment.
Answer D. The change is 100 points, not more than 100. The named condition is strict. This arithmetic does not rule out other competitive concerns or establish the final legal result.
Answer A. Square and add the percentage-point shares: 40² + 30² + 20² + 10² = 1,600 + 900 + 400 + 100 = 3,000. Using decimal shares gives 0.30, which is on a different scale and must not be compared directly with a threshold such as 1,800.
Connect Concentration to Economic Reasoning
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