Kenteli

What ETF overlap is, and why it hides

Guide · 24 September 2026

Overlap is the part of your portfolio you own more than once. Two funds, bought on different days for different reasons, turn out to hold many of the same companies. Your money is spread across fewer businesses than the number of tickers suggests.

Four funds can be one decision

Picture a portfolio with a broad market fund, a technology fund, a large-cap fund and something described as an income fund. That looks like four decisions. Underneath, the first three may be buying the same handful of very large companies, in slightly different proportions. The fourth is probably doing something genuinely different, which is exactly the point: you cannot tell which is which from the names.

This is not a flaw in the funds. A total market fund is supposed to hold almost everything, and the biggest companies are the biggest part of almost every index that includes them. The fund did what it said. The duplication appears when you own several such funds at once, and nobody is responsible for telling you what they add up to.

Why nothing on your screen shows it

Open a brokerage account and you see a balance, a list of positions, and some measure of return. None of those three can reveal overlap, and it is worth being precise about why.

• A balance is one number. It says what everything is worth, not what it is made of.
• A position list stops at the fund. It tells you that you hold a fund, not that the fund holds two hundred companies, forty of which sit inside another fund you also hold.
• A return figure reports what happened, not what you are exposed to. Overlap changes the range of things that could happen next, which is not a quantity last month's performance can express.

Each fund does publish its holdings. The work nobody does for you is the joining up: taking every fund you own, weighting each one by how much of it you actually hold, and adding the same company's slices together.

What it costs you

The honest answer is that overlap is not automatically bad. Someone who deliberately wants a large position in the biggest companies and gets there through two funds has not made a mistake. They have made a choice, and it is a choice they can defend.

The problem is the version nobody chose. That is where diversification is assumed rather than measured, and the portfolio is more concentrated than its owner believes. When one company or one sector has a bad year, the portfolio moves more than expected, and the surprise arrives at the worst possible time. A second cost is quieter. If two funds give you substantially the same exposure, you may also be paying the expenses of both funds for exposure that is more duplicated than you intended.

How you actually see it

You look through the funds to the companies underneath. In practice that means four steps, and they are tedious by hand, which is why most people never do them.

• Get the current holdings and weights for every fund you own.
• Scale each fund's weights by how much of that fund you hold, so a small position does not count the same as a large one.
• Add up every company across all funds, including shares you hold directly.
• Sort by size, and see what is really at the top of your portfolio.

Most people expect the result to be a long tail of hundreds of names. It usually is. What surprises them is the head of the list, and how much of the total sits in the first handful of companies.

How much overlap is too much

There is no threshold that is true for everyone, and anyone who gives you one without asking what you are trying to do is guessing. Two funds that mostly hold the same companies are fine if you wanted that exposure and knew you were buying it twice. The same two funds are a problem if you bought the second believing it would balance the first.

A more useful question than "how much" is "on purpose or by accident". You can only answer it once you can see the number.