ETF and Mutual Fund Overlap: Look-Through Across Funds and Fund of Funds

The previous guide in this series covered Form N-PORT and what a single fund owns as of a single month-end. Few portfolios hold just one fund. An allocator may hold a developed-markets index ETF in one sleeve and an international core fund in another. A wealth platform may show a client three funds that look different on a statement and own many of the same stocks. A risk team may see a target-date fund as one line while the money sits in five other funds underneath it.

Fund overlap measures how much two funds own in common. Look-through replaces a position in a fund with the securities that fund holds. The two depend on each other. A fund that holds other funds will show almost no overlap with anything until someone looks through it, and the overlap figure that comes out depends on choices that are easy to miss: which denominator, which identifier, which report date.

This guide covers the main ways to measure ETF and mutual fund overlap, why one pair of funds can score 26%, 85% or 100% depending on the method, how fund-of-funds structures work under SEC Rule 12d1-4, and what it takes to look through more than one layer of funds when each layer reports on its own schedule.

What fund overlap is and how to measure it

Fund overlap is the share of one fund's portfolio that also appears in another. How much depends on the counting method, and the two common methods answer different questions.

The first is count overlap: the number of securities both funds hold, divided by the number of securities one of them holds. Because the two funds usually hold different numbers of securities, count overlap has two answers for every pair, one from each side. Take two iShares funds that track MSCI indexes of developed markets outside North America. As of August 26, 2026, iShares MSCI EAFE ETF (EFA) held 673 stocks, and every one of them also appeared in iShares Core MSCI EAFE ETF (IEFA), which held 2,615. Measured from EFA's side, the overlap is 100%. Measured from IEFA's side, it is 673 out of 2,615, or about 26%. Both figures are correct, and a report that shows only one of them leaves the reader to guess which.

The second is weight overlap. For each security both funds hold, take the smaller of the two portfolio weights, then add those smaller weights together. The result is the portion of each fund's assets that matches the other fund position for position, and it comes out the same from either side. For EFA and IEFA on the same date, weight overlap was 85.4%. The 1,942 stocks that only IEFA held were mostly smaller companies, the difference between a standard index and its broader version, and together they made up about 13% of IEFA's net assets. Count overlap from IEFA's side made the two funds look far apart. Weight overlap showed that most of the money was invested the same way.

Weight overlap has a close relative that many investment teams already use. Active share, introduced by Martijn Cremers and Antti Petajisto in a 2009 paper in The Review of Financial Studies, measures how much a fund's holdings differ from its benchmark: half the sum of the absolute differences in weight across every security either one holds. When both portfolios are fully invested and their weights sum to 100%, weight overlap equals 100% minus active share. Treating one fund as the benchmark for the other gives the same answer from a different direction. In practice the weights rarely sum to exactly 100%, because funds hold cash, futures and other instruments alongside their securities, so the calculation should state whether weights were rescaled before comparing.

Neither measure tells an allocator how much exposure is doubled up in dollars. For that, combine the positions at the security level, weighting each fund's holdings by the amount actually invested in it, and look at the largest names in the combined book. Overlap is a summary statistic. The combined holdings are what a concentration limit or an exposure report will be measured against.

Why funds overlap more than their names suggest

Most overlap between funds traces back to how indexes are built. Index providers publish families of related indexes: a standard version and a broader one that adds smaller companies, regional slices of a global index, and growth, value, factor and ESG variants that start from the same parent universe. Funds that track members of one family will share holdings by design. EFA and IEFA are a clean example, since one index is largely contained in the other, and the same pattern runs through regional, style and screened funds across fund families.

Market-cap weighting pushes weight overlap higher still. In a cap-weighted fund, the largest companies carry most of the weight, and those same companies sit near the top of almost every broad fund in the same market. Two funds can hold hundreds of different small companies and still share most of their assets, because the small positions contribute little weight. This is why count overlap and weight overlap so often disagree, and why weight overlap is usually the better guide to how differently two funds will behave.

Active funds overlap too. Managers in the same category draw from the same opportunity set, and many hold the large names in their benchmark at or near benchmark weight. A fund with low active share against its benchmark will show high weight overlap with an index fund tracking that benchmark, which is worth knowing before paying an active fee for both. Holding more funds in this situation adds cost and monitoring work without adding much diversification, the pattern investors sometimes call "diworsification".

Bond funds raise a different problem. Two bond funds rarely hold the same individual bonds, because issuers sell many separate issues with different maturities, coupons and identifiers. Measured at the security level, two core bond funds can look almost unrelated while lending to many of the same issuers. For fixed income, overlap is more useful measured at the issuer level, which means rolling each bond up to its issuer, and ideally to the issuer's parent, before comparing. That roll-up needs reliable issuer identifiers such as the Legal Entity Identifier, and it is where much of the work in bond fund overlap sits.

What is a fund of funds?

A fund of funds is a fund that invests some or all of its assets in other funds. Target-date funds are the most familiar example: a single fund that holds a set of underlying stock and bond funds and shifts the mix over time. Multi-asset allocation funds, balanced funds and many model-portfolio products work the same way. So do some ETFs that hold other ETFs, including currency-hedged funds that own an unhedged fund and add currency forwards on top.

The structure is common. In the release adopting Rule 12d1-4 in 2020, SEC staff estimated that about 40% of registered funds held an investment in at least one other fund, and that 26% of those held more than 90% of their assets in other funds. The same release cited Investment Company Institute figures showing assets in mutual funds that invest primarily in other mutual funds growing from $469 billion in 2008 to $2.54 trillion in 2019. Those figures are several years old and the current numbers are likely different.

The limits in Section 12(d)(1) and Rule 12d1-4

The Investment Company Act limits how much one fund can own of another. Under Section 12(d)(1)(A), a registered fund generally cannot acquire more than 3% of another fund's outstanding voting securities, invest more than 5% of its total assets in any one fund, or invest more than 10% of its total assets in funds overall. Congress set these limits to address three concerns: an acquiring fund using its stake to control the fund it buys, layers of duplicated fees, and structures too complex for investors to follow.

For decades, funds of funds operated under a mix of statutory exceptions, SEC rules and individual exemptive orders. Rule 12d1-4, adopted on October 7, 2020 and effective January 19, 2021, replaced much of that with one framework. It lets registered funds and business development companies invest in other registered funds and BDCs beyond the Section 12(d)(1) limits, subject to conditions. The acquiring fund and its advisory group cannot control the acquired fund. Above set ownership levels they must vote their shares in proportion to other holders. The advisers must make findings about the arrangement, including that fees are not duplicated, and funds with different advisers must sign a fund of funds investment agreement. The rule also generally bars three-tier structures, with exceptions that include letting an acquired fund put up to 10% of its assets in other funds. Private funds and foreign funds cannot rely on the rule as acquiring funds.

What the structure means for disclosure and cost

For anyone measuring overlap, the important consequence is what a fund of funds reports. Its N-PORT filing or its sponsor's holdings file lists the funds it owns. The stocks and bonds inside those funds appear only in the underlying funds' own reports. A target-date fund's holdings can therefore show five or six lines, each standing in for thousands of securities.

Cost works in layers too. Form N-1A requires a fund's prospectus fee table to show acquired fund fees and expenses as a separate line when the fees of the funds it holds exceed one basis point of its average net assets. Fee benchmarking that reads only the top-level expense ratio can understate what a fund of funds costs, a topic later in this series covers in more detail.

Fund of funds look-through in practice

Looking through a fund of funds means replacing each position in another fund with that fund's own holdings, scaled by the size of the position. If a parent fund holds 30% of its assets in a stock fund, and that stock fund holds 2% in a given company, the parent's look-through exposure to the company through that position is 30% times 2%, or 0.6%. Add the same calculation across every underlying fund, plus anything the parent holds directly, and the result is the parent's exposure security by security.

A currency-hedged ETF shows why this step matters for overlap. iShares Currency Hedged MSCI EAFE ETF (HEFA) gets its stock exposure by owning EFA. On August 26, 2026, its holdings file listed one stock position, EFA shares at 100.87% of net assets, alongside currency forwards and a small amount of cash. Compared security by security with IEFA as filed, HEFA has nothing in common with it: the only equity line it reports is another fund, and IEFA does not hold that fund. Measured overlap is 0%. Looked through, HEFA holds EFA's 673 stocks at 100.87% of EFA's weights, and its weight overlap with IEFA is 85.4%, the same as EFA's. An overlap screen that skips the look-through would tell an allocator holding HEFA and IEFA that the two funds are unrelated, when most of the money is in the same stocks.