How do I calculate my true exposure to a single stock across funds?

Multiply each fund's weight in your portfolio by that stock's weight inside the fund, then sum across funds. A company at 8% of one fund and 6% of another, with half your money in each, is 7% of your total portfolio — a figure neither fact sheet shows.

Does holding more funds mean more diversification?

Not necessarily. Funds drawn from the same universe on similar criteria hold the same large companies, so additional funds often add fee and administration without adding exposure. Diversification is a property of the underlying holdings, not of the number of products.

Why does the number of stocks in a portfolio not measure risk?

Because the long tail contributes almost nothing. A 0.2% position must move fifty per cent to shift the portfolio by a tenth of a per cent, so a large holding count mostly describes the tail while the top ten positions determine the outcome.

Lens · 16 September 2026

Ten funds, one bet: the concentration behind a diversified portfolio

Holding more funds spreads the fee, not necessarily the risk. How to compute what you actually own at the stock level, why the top holdings dominate, and what a genuine reduction in concentration looks like.

2 min read

Diversification is a property of what you own, not of how many products you bought to own it. Ten funds drawn from the same universe, screened on similar criteria, holding the same large companies, is one position wearing ten labels — and the fee is the only thing that has genuinely been diversified.

Look through the fund to the stock

The calculation that matters is a look-through: for every stock, the sum of what you hold in it across every fund, weighted by how much of your money sits in each fund.

exposure(stock) = Σ over funds weight(fund in portfolio) × weight(stock in fund)

Both weights are fractions of one. The result is the share of your total portfolio riding on that one company, regardless of how many funds it arrived through.

Worked example

Half your money is in Fund A, half in Fund B. Company X is 8% of Fund A and 6% of Fund B.

0.5 × 8% + 0.5 × 6% = 7%

Seven per cent of the whole portfolio is in one company — and neither fund's own fact sheet shows a number anywhere near that as a portfolio-level risk.

Illustrative shape, not a real portfolio. What is worth taking is that the long tail contributes very little to the outcome — a position of a tenth of a per cent moves nothing, however many of them there are.

Why the tail does so little

A holding of 0.2% has to move fifty per cent to shift the portfolio by a tenth of a per cent. Three hundred such positions do not add three hundred bets; they add a small, expensive approximation of the index the large positions already track.

What actually reduces concentration

Adding an asset that behaves differently — not another fund selecting from the same universe on similar criteria.
Checking the overlap between candidates before buying rather than after, since two funds with high overlap add fee and admin without adding exposure.
Sizing at the look-through level: deciding what share of the whole portfolio any one company may represent, and holding to it.
Re-running the calculation periodically. Concentration drifts upward on its own as winners grow, without a single transaction.

None of this argues for fewer holdings as such. It argues for knowing what you hold — which is a different exercise from counting the products you hold it through, and usually a more uncomfortable one.

Common questions

How do I calculate my true exposure to a single stock across funds?: Multiply each fund's weight in your portfolio by that stock's weight inside the fund, then sum across funds. A company at 8% of one fund and 6% of another, with half your money in each, is 7% of your total portfolio — a figure neither fact sheet shows.
Does holding more funds mean more diversification?: Not necessarily. Funds drawn from the same universe on similar criteria hold the same large companies, so additional funds often add fee and administration without adding exposure. Diversification is a property of the underlying holdings, not of the number of products.
Why does the number of stocks in a portfolio not measure risk?: Because the long tail contributes almost nothing. A 0.2% position must move fifty per cent to shift the portfolio by a tenth of a per cent, so a large holding count mostly describes the tail while the top ten positions determine the outcome.

Mutual fund portfolio overlap: how to measure what you actually own

Holding five funds does not mean holding five portfolios. How to compute overlap between funds, what counts as high, and why diversification by fund count is a poor proxy for diversification.

3 min read

Rolling returns: the number that survives a different start date

A point-to-point return is one draw from a distribution, and the draw is chosen by the start date. What rolling returns measure instead, how to read the spread, and why the median matters more than the headline.

3 min read

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