Why is the figure a lower bound rather than the real overlap?

Because it can only see what the funds disclose. Top-holdings disclosure covers the largest positions and stops, so two funds sharing hundreds of smaller names identically will show only the overlap among the ones both happen to publish. Nothing outside that set can reduce the overlap, so the true figure is always at least what is reported and usually well above it.

Two funds track the same index but do not show 100% — why?

Disclosure timing and sampling. The two funds published their holdings on different dates, may hold slightly different numbers of positions, and may weight them marginally differently at the margins. For two funds on the same index the honest reading is that they are substantially identical regardless of what the percentage says.

Is overlap a problem?

Only if you did not intend it. Deliberately overweighting a region by holding a broad fund and a regional one is a position, not a mistake. It becomes a problem when the second fund was bought in the belief that it was adding diversification, since you then carry two funds' worth of cost and admin for one fund's worth of exposure.

ETF overlap checker: how much of two funds is the same company

Adding a second fund only diversifies you if it holds something the first one does not. The pairing that most often disappoints is a world tracker plus an S&P 500 tracker, because a world index is already well over half US large caps — the second fund mostly buys more of what the first fund already owned, at a higher concentration.

The calculator

How it works

For every company held by both funds the calculator takes the smaller of the two weights and adds those up. Taking the minimum is the whole point: if a company is 4% of one fund and 2% of the other, only 2% of your money is genuinely doubled up, and counting more than that would overstate the concentration. The result is the standard overlap coefficient, alongside the shared names and what each fund holds that the other does not.

What it assumes

This runs on publicly disclosed top holdings, which is a small slice of a fund that may hold hundreds or thousands of positions. That makes the reported figure a lower bound rather than an estimate: every shared position outside the disclosed set is invisible here, and each one would push the true overlap up, never down. Read the number as at least this much, and treat the disclosure dates as approximate.

Common questions

Why is the figure a lower bound rather than the real overlap?: Because it can only see what the funds disclose. Top-holdings disclosure covers the largest positions and stops, so two funds sharing hundreds of smaller names identically will show only the overlap among the ones both happen to publish. Nothing outside that set can reduce the overlap, so the true figure is always at least what is reported and usually well above it.
Two funds track the same index but do not show 100% — why?: Disclosure timing and sampling. The two funds published their holdings on different dates, may hold slightly different numbers of positions, and may weight them marginally differently at the margins. For two funds on the same index the honest reading is that they are substantially identical regardless of what the percentage says.
Is overlap a problem?: Only if you did not intend it. Deliberately overweighting a region by holding a broad fund and a regional one is a position, not a mistake. It becomes a problem when the second fund was bought in the belief that it was adding diversification, since you then carry two funds' worth of cost and admin for one fund's worth of exposure.

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ETF overlap checker: how much of two funds is the same company