ETF overlap: how much of your second fund did you already own?

You buy a global ETF for broad exposure. Later you add an S&P 500 ETF, because the US has been doing the heavy lifting and you want more of it. Two funds, two different names, two different indices. It feels like diversification.

Then you look inside them, and most of the second fund is the first one again.

How much do an S&P 500 fund and a global fund actually share?

Here is the measurement, taken on 14 August 2026 from the issuer's own published holdings files.

The two funds hold 455 of the same companies. That is not the interesting part. The interesting part is the last row, where the same 455 stocks account for almost all of one fund and roughly two thirds of the other.

Why the overlap is asymmetric

An overlap figure is usually quoted as one number, and that hides what is actually going on.

The S&P 500 fund is nearly a subset of the global fund. Almost everything it owns, 98.8% by weight, is also sitting in the global fund. The global fund is the broader container: it holds those same US large caps and then roughly 800 more companies from Japan, the UK, Canada, and the rest of the developed world.

So the direction matters:

• If you already hold the global fund and add the S&P 500 fund, you are adding almost no new companies. You are increasing the weight of names you already owned.
• If you already hold the S&P 500 fund and add the global fund, you are genuinely adding something, because roughly 31% of the global fund by weight is companies the S&P fund does not hold at all.

Same pair of funds. Completely different answer depending on which one came first.

What this looks like at the individual stock level

The shared names are not a long tail of small positions. They are the largest positions in both funds, at different weights.

Every one of these appears in both funds. If you hold both, your actual position in each company is a blend of the two weights, and it is higher than either row suggests on its own. Nobody sets out to put a specific percentage of their savings into one chip designer. It arrives as a by-product of owning two funds that both track the same market from different angles.

This is the same concentration question the ETF composition guide covers for a single fund, except it compounds when funds are stacked.

How to check the overlap between your own funds

You do not need a tool or a subscription for this. The large issuers publish full daily holdings, free, on the product page for each fund.

1. Find the fund's product page on the issuer's site. Search the ticker or the ISIN, not the marketing name, because near identical names exist across share classes.
2. Download the holdings file. It lists every position with its ticker, ISIN, weight, sector and country.
3. Do the same for the second fund.
4. Match on ISIN rather than ticker. The same company can carry different ticker strings on different exchanges, and ISIN is unique. Ticker matching is quicker and usually close enough, but it will miss a few dual-listed names.
5. Add up the weight of the matches, in each fund separately. Two numbers, not one. That asymmetry is the answer.

Two things worth knowing before you interpret what you get. Weights move with prices, so these figures drift week to week, and the ones in this article are a snapshot of one day rather than a constant. And the weights in a holdings file will not always sum to exactly 100%, because funds also hold cash and instruments that do not carry an equity ticker.

The same fund under two names

One more trap, and it is a cheaper mistake to avoid than the overlap itself.

SXR8 and CSPX in the table above are not two funds. They are the same iShares Core S&P 500 UCITS ETF, listed on different exchanges and quoted in different currencies. IWDA and SWDA are likewise one fund. A portfolio holding both SXR8 and CSPX has no diversification between them whatsoever, because there is nothing between them.

Before comparing two funds, check the ISIN of each. If the ISIN matches, the overlap is 100% and no further analysis is needed.

What the overlap number does and does not tell you

It tells you what you own. That is genuinely useful, and most people holding several funds have never added it up.

It does not tell you what to do about it. Whether a concentrated position is appropriate depends on why each fund is in the portfolio, and that is not something an article can determine from the outside. What the measurement does is replace an assumption with a figure, so any decision that follows is made against the real composition instead of against two fund names that sound different.

If you want the same treatment applied to individual companies, the fundamentals guide covers what to check inside a single business, and you can run an analysis on any of the names above to see what sits behind the weight.

The same measurement, run every week across the largest US equity ETFs, is published as a table on the ETF overlap page: which pairs share the most weight, and which stocks sit in the most funds. The US-listed counterparts of the two funds above have their own pages built from their SEC filings, IVV for the S&P 500 and URTH for MSCI World, and the pair itself is measured at IVV vs URTH.