ETF Concentration Risk: How to Measure It

Key Takeaways

Direct answer: ETF concentration risk is the risk that a small number of holdings, issuers, sectors, or shared drivers determines the fund’s result. Measure it directly from the fund’s daily holdings file rather than from its name or its diversification label: group every line onto its parent issuer, then read the largest single weight, the top 10 combined weight, the effective number of holdings, and the largest sector weight. The legal definitions are much weaker than the everyday word implies. Under the Investment Company Act, a diversified company only has to keep 75 percent of total assets within a 5 percent per-issuer limit, and a regulated investment company can put up to 25 percent of total assets in one issuer.

• "Diversified" is a defined statutory term, not a description. 15 U.S.C. 80a-5(b)(1) defines a diversified company as one where at least 75 percent of total assets is in cash, government securities, other investment companies, and other securities limited to 5 percent of total assets per issuer and 10 percent of an issuer’s voting securities. The other 25 percent has no per-issuer cap from that provision.
• The status survives drift. 15 U.S.C. 80a-5(c) provides that a registered diversified company does not lose that status because of a later discrepancy between its holdings and the test, so long as the discrepancy is not the result of an acquisition. A position that grows from 4 percent to 12 percent through appreciation alone breaks nothing.
• The tax test is looser still. Under 26 U.S.C. 851(b)(3), a regulated investment company needs only 50 percent of total assets inside the 5 percent and 10 percent limits, and separately must keep no more than 25 percent of total assets invested in any one issuer.
• Line count is not issuer count. Multiple share classes and multiple bond series of the same company appear as separate rows with separate identifiers, so a naive count understates single-name exposure.
• Effective number of holdings is a better summary than raw holdings count. A 500-line fund whose weights are dominated by a handful of names can behave like a 40-position portfolio.
• Concentration and correlation are different problems. Ten positions in ten separate companies that all rise and fall on the same input are a correlation problem, and no per-name weight cap detects it.
• Concentration is not automatically bad. It is the mechanism by which an active or thematic fund can outperform. The failure is unmeasured concentration, or concentration that contradicts the role the fund plays in the portfolio.

What Is ETF Concentration Risk?

Concentration risk is the sensitivity of a portfolio’s outcome to a small number of its parts. In an ETF the parts can be defined at several levels, and a fund can be concentrated at one level while looking well spread at another:

• Single name. One security is a large share of the fund.
• Issuer. One company is a large share of the fund across several securities: two share classes, a preferred, and a bond, for example.
• Sector or industry. A group of different companies whose fortunes move on the same demand cycle.
• Geography or currency. A group of holdings exposed to one economy, one policy regime, or one exchange rate.
• Factor. A group of holdings that share a statistical characteristic such as growth, momentum, or leverage, regardless of what industry they are in.
• Counterparty or structure. Exposure obtained synthetically through one swap counterparty, or collateral held with one custodian.

An investor holding a single broad-market ETF is normally worried about the first three. An investor holding four or five funds is usually worried about the fourth and fifth, because the concentration builds up between funds rather than inside any one of them. That second problem is measured with weighted overlap and look-through exposure, and the ETF overlap analyzer is the tool that does that arithmetic. This guide handles the first problem: concentration inside a single fund.

The related but separate question of how concentrated the market itself has become, and what that does to index breadth, belongs to the technical-analysis discipline rather than to fund research. If your question is about market leadership rather than fund construction, index concentration is the page that measures it, and the two lenses are worth keeping distinct: a broad index fund can be perfectly constructed and still be concentrated because the index it tracks is.

What Do the Diversification Labels Actually Guarantee?

Two separate legal tests get compressed into the single word "diversified" in everyday use. Neither one does what most investors assume.

The first is the Investment Company Act subclassification. 15 U.S.C. 80a-5(b)(1) defines a diversified company as a management company where at least 75 percent of the value of its total assets is represented by cash and cash items including receivables, government securities, securities of other investment companies, and other securities limited, for that calculation, to no more than 5 percent of total assets per issuer and no more than 10 percent of that issuer’s outstanding voting securities. A non-diversified company is defined simply as any management company that is not a diversified company.

Read that carefully and the gap appears immediately. The test constrains 75 percent of assets. The remaining 25 percent is unconstrained by that provision, so a fund can be a diversified company while a single position occupies a quarter of the portfolio.

The second is the tax test that lets a fund avoid entity-level taxation. 26 U.S.C. 851(b)(3) requires, at the close of each quarter, that at least 50 percent of the value of total assets is represented by cash, government securities, securities of other regulated investment companies, and other securities limited to 5 percent of total assets per issuer and 10 percent of the issuer’s outstanding voting securities, and separately that not more than 25 percent of the value of total assets is invested in the securities of any one issuer, of two or more issuers the taxpayer controls that are engaged in the same or similar or related trades or businesses, or of certain publicly traded partnerships.

Then there is the drift provision, which is the part that surprises people most. 15 U.S.C. 80a-5(c) provides that a registered diversified company that met the test when it qualified does not lose diversified status because of a subsequent discrepancy between the value of its investments and the requirements of that paragraph, as long as the discrepancy immediately after any acquisition of a security or other property is not wholly or partly the result of that acquisition.

In plain terms: the limits bind at purchase, not thereafter. A market-capitalization-weighted index fund that bought a position at 3 percent and watched it compound to 11 percent has not breached anything, and nothing forces it to trim. That is the correct design for an index fund, and it is exactly why the label cannot substitute for measurement.

How Do You Measure Concentration in an ETF?

All of the measures below come straight out of the daily holdings file that Rule 6c-11 requires every ETF to publish, so none of them requires a data subscription. If you have not worked with that file before, how to read an ETF holdings file covers the fields and the traps first.

The effective number of holdings is the measure worth adding if you currently use only one. It answers a question the raw count cannot: how many equally weighted positions would produce the same concentration as this fund’s actual weights? Compute it by converting each weight to a decimal, squaring each one, summing the squares, and taking the reciprocal. A fund with 400 lines and an effective number of 45 is not a 400-stock portfolio in any way that matters to risk. The same arithmetic, scaled differently, is the Herfindahl-Hirschman index used in industry structure work; market concentration and the HHI covers that application.

One measurement rule matters more than the choice of formula: group by issuer before you compute anything. A holdings file lists instruments. Risk attaches to companies. Two share classes of the same company, or a company’s equity alongside its bonds in a multi-asset fund, are one exposure appearing as several rows.

Worked Example: Two Funds With the Same Holdings Count

The two hypothetical funds below were constructed for this guide. Both hold 100 issuers. Both would satisfy the diversified-company test. Their concentration profiles are not comparable.

Fund A’s top 10 are modelled at 9.0, 7.0, 6.0, 5.0, 4.5, 4.0, 3.5, 2.5, 2.0 and 1.5 percent, summing to 45.0 percent, with the remaining 55.0 percent spread evenly across 90 issuers at 0.6111 percent each. Squaring and summing those weights gives 0.02475 from the top 10 and 0.00336 from the tail, for 0.02811 in total. The reciprocal is 35.6. Fund B’s 100 equal 1 percent weights each square to 0.0001, summing to 0.0100, whose reciprocal is exactly 100.

Two conclusions follow, and neither is visible from the holdings count.

1. Fund A behaves like a 36-stock portfolio. Nearly two thirds of its concentration comes from ten positions. A serious problem at its largest holding moves the fund roughly nine times as much as a problem at an average holding in Fund B.
2. Fund B has traded name concentration for something else. Equal weighting forces persistent selling of what has risen and buying of what has fallen, which raises turnover and shifts the fund’s size profile toward smaller companies. That is a different risk, not an absence of risk, and it shows up in trading costs and in factor exposure rather than in a top-10 table.

Every figure above was computed for this illustration from the stated weights and can be reproduced. Neither fund is real, and nothing here is a forecast or a recommendation.

Where Concentration Hides

The measures above catch weight concentration. Several important forms of concentration produce a clean weight table and still decide the outcome.

• Shared input or customer. Twenty separate companies at 2 percent each, all of whose margins depend on the same commodity price or the same handful of buyers, is a 40 percent position in that input. No per-name cap sees it.
• Classification boundaries. A company can be classified into a sector whose label does not describe what drives its revenue. Sector weight is only as good as the classification scheme behind it, which is why the GICS taxonomy and how to use it is worth reading before trusting a sector table.
• Index methodology caps that bind only at rebalance. Capped index funds constrain weights at reconstitution dates. Between those dates the cap is not enforced, so a fund can carry an above-cap position for weeks legitimately.
• Concentration between your funds rather than inside them. Three well-diversified funds can each be fine and still deliver a portfolio where one company is your largest position by a wide margin. That is look-through exposure, and it is the specific job of the ETF overlap analyzer.
• Structural concentration. Synthetic exposure through a small number of swap counterparties, or a securities-lending programme concentrated with one borrower, does not appear as a large equity weight anywhere.
• Correlation that only appears under stress. Diversification measured on calm-period data overstates itself precisely when it is needed. That is a stress-testing question rather than a weights question, covered in correlation breakdown in crises.

The SEC’s own investor education makes the underlying point about behaviour rather than measurement: its Beginners’ Guide to Asset Allocation, Diversification, and Rebalancing notes that savvy investors typically do not change their asset allocation based on the relative performance of asset categories, for example by increasing the proportion of stocks when the stock market is hot, and instead rebalance. Concentration that has built up through appreciation is the most common thing an investor discovers at exactly the moment they are least willing to act on it.

What Counts as Too Concentrated?

There is no universally correct threshold, and any page that gives you one is inventing it. What can be stated precisely is the relationship between a fund’s role in your portfolio and the concentration that role can tolerate.

The practical discipline is to write the limit down before you look at the number. A stated position policy converts a vague discomfort into a testable rule, and the machinery for that already exists in position caps and maximum exposure rules, which covers how a cap is set, measured, and enforced at the portfolio level rather than the fund level.

Two further notes on interpretation. First, a fund being more concentrated than its benchmark is a fact, not a verdict; the verdict depends on whether you were paying for benchmark replication. Second, concentration measured on a single day is a snapshot. A fund whose top 10 weight has climbed steadily across several quarters is telling you something a one-day reading cannot, which is why the measurement is worth repeating on a schedule rather than once at purchase.

A Concentration Check You Can Run in Ten Minutes

1. Download the fund’s current daily holdings file and confirm the as-of date is the prior business day.
2. Confirm you have the whole file, by comparing the row count to the fund’s stated number of holdings. A top-holdings extract will produce confidently wrong numbers.
3. Collapse rows onto parent issuers, combining share classes and multiple instrument types.
4. Record the largest issuer weight and the top 10 combined weight.
5. Compute the effective number of holdings as the reciprocal of the sum of squared decimal weights, and compare it to the raw issuer count.
6. Group by sector and record the largest sector weight, noting which classification scheme the issuer used.
7. Compare every number above to the fund’s stated benchmark, not to a general expectation. A fund tracking a concentrated index should be concentrated.
8. Repeat across every fund you hold and aggregate at issuer level. This is where most real concentration is found.
9. Write down the threshold that would make you act, and the action, before the next review.

Steps 1 through 6 use only the free daily file. Step 8 is the one that changes decisions most often, and it is the step almost nobody does, because it requires combining files rather than reading one.

Common Mistakes and Misconceptions

• Reading "diversified" as a promise of broad exposure. It is a statutory subclassification with a 75 percent scope and a 25 percent unconstrained remainder.
• Assuming a large holdings count means low concentration. The effective number of holdings routinely lands at a small fraction of the raw count.
• Counting rows instead of issuers. Share classes and multiple bond series split one exposure across several lines.
• Believing a fund must trim a position that breaches a limit. The Investment Company Act test binds at acquisition, and 80a-5(c) expressly preserves status through subsequent drift.
• Treating concentration as automatically bad. A thematic fund that is not concentrated has failed at its job. The failure mode is unmeasured concentration, or concentration inconsistent with the fund’s role.
• Measuring inside funds but never across them. Portfolio-level look-through is where the largest single-name exposures usually turn up.
• Confusing concentration with correlation. Weight caps do not detect shared drivers, and calm-period correlation understates crisis behaviour.
• Comparing sector weights across providers without checking the classification scheme. Different schemes assign the same company to different sectors.

Concentration Is a Measurement, Not a Label

The most useful thing an investor can take from this topic is that the words on a fund’s page and the numbers in its holdings file answer different questions. "Diversified" answers a legal question about how the fund was constructed at the moment it bought its positions. The holdings file answers the question you actually care about: if one company, one industry, or one shared driver goes badly wrong, how much of this fund goes with it. Those two answers can diverge dramatically, and the divergence widens in exactly the market conditions that make concentration matter, because a long run of strong performance in a narrow group of companies grows their weights without any purchase taking place and without any limit being breached.

The measurement itself is not hard. Group by issuer, take the largest weight, take the top 10, compute the effective number of holdings, take the largest sector weight, and compare all five to the benchmark the fund claims to follow rather than to a rule of thumb. Every one of those numbers comes out of a file that Rule 6c-11 requires the sponsor to publish, free, before the market opens. The barrier is not access to data or to formulas. It is the discipline of collapsing instrument rows onto companies, and of repeating the exercise across the whole portfolio rather than one fund at a time.

The final judgement is not a number at all. It is whether the concentration you measured is the concentration you meant to own. A single-country technology fund that turns out to be dominated by three companies is doing what it was built to do, and the honest question is whether you sized it accordingly. A core holding described as broad-market exposure that turns out to behave like a 35-stock portfolio is a different conversation, and it is better to have that conversation from a spreadsheet on an ordinary Tuesday than from a headline. Write the limit down first, measure against it on a schedule, and aggregate across funds, because the exposure that eventually matters is almost never the one inside any single fund.

Frequently Asked Questions

References

This guide is based on the operative United States statutes and SEC materials, each retrieved and verified on 22 August 2026:

• Cornell Legal Information Institute: 15 U.S.C. 80a-5, Subclassification of management companies: the definition of a diversified company in subsection (b)(1) with its 75 percent, 5 percent, and 10 percent thresholds, the definition of a non-diversified company in subsection (b)(2), and the loss-of-status provision in subsection (c) that preserves diversified status through subsequent drift.
• Cornell Legal Information Institute: 26 U.S.C. 851, Definition of regulated investment company: the diversification requirements in subsection (b)(3), including the 50 percent test with its 5 percent and 10 percent per-issuer limits and the separate 25 percent single-issuer ceiling.
• eCFR: 17 CFR 270.6c-11, Exchange-traded funds: the daily website disclosure condition that makes every measurement in this guide possible without a data subscription.
• SEC: Beginners’ Guide to Asset Allocation, Diversification, and Rebalancing: the observation that investors typically rebalance rather than shift allocation toward whichever asset category has recently performed well.

The two-fund comparison in this guide is an original, hypothetical illustration. Both weight distributions were specified for this page and every derived figure, including the sums of squared weights and the effective numbers of holdings, was computed from those stated weights and can be reproduced from the table. Neither fund exists, and no figure here is a quoted price, a projection, or a recommendation. This is educational content, not personalized investment, tax, or legal advice.