How Much of Your Portfolio Should Be in One Stock?

There is no single legal or universal limit, but the working conventions cluster tightly. Most practitioners treat 5% as the comfortable ceiling for a single stock, 10% as the line where a position starts driving portfolio outcomes, and 20% as the point where you no longer own a diversified portfolio, you own that company with some other holdings attached. Anything past 30% is a concentrated bet, which is a legitimate choice as long as it is a choice.

The reason the published guidance ranges from 5% to 20% is not that professionals disagree about the math. It is that they are answering slightly different questions: how much can one holding wobble your year, how much can you afford to lose entirely, and how much tax stands between you and trimming it. Everything below is educational content, not investment advice.

What percentage of my portfolio should be in one stock?

Five percent is the most defensible default for a portfolio of individual stocks. At 5%, a holding that goes to zero costs you a bad quarter rather than a bad decade, and a holding that triples still meaningfully improves your result. That asymmetry is the whole argument. Fidelity's own portfolio analysis tooling flags single positions above 5% for exactly this reason, and the number is common in institutional policy documents too.

The practical caveat is that 5% implies at least 20 positions if you are fully invested in individual names, and most self-directed investors do not want to follow 20 companies properly. If you would rather hold 10 to 12 names you actually understand, your natural position size is closer to 8% to 10%, and the honest trade is that you are accepting more single-name risk in exchange for a portfolio you can genuinely keep up with. That is a reasonable trade. It is only a problem when it happens by accident.

How much is too much concentration in one stock?

Twenty percent is where most guidance stops hedging. J.P. Morgan Private Bank, T. Rowe Price and most wealth managers publish material on concentrated equity positions aimed squarely at people past that mark, and the framing is consistent: past roughly 20%, the single holding, not the market and not your other decisions, determines your outcome. Above 30% the position is speculative in the plain sense of the word, however good the company is.

Is 10% in one stock too much?

Not automatically, but it is the threshold where a position stops being one of many and starts steering the portfolio. At 10%, a stock that halves costs you 5% of everything you own, which is roughly an average market correction delivered by a single company. If you hold at 10% deliberately, understand the business, and can name what would make you sell, that is an informed concentration. If you hold at 10% because it grew there from 4% and you never looked, that is drift wearing the costume of conviction.

The useful test is prospective rather than backward looking. Ask what you would do if the position fell 50% next month. If the honest answer is that you would be forced to sell to sleep at night, the position is already too large, because a size you cannot hold through a drawdown is a size that will get liquidated at the worst possible moment.

How much company stock is too much?

Employer stock deserves a stricter limit than any other holding, and most guidance lands at 10% to 15% maximum. The reason is that the risk is doubled up rather than merely concentrated. Your salary, your health coverage, your equity vesting schedule and your position are all claims on the same company. When it goes badly, the stock falls in the same quarter your job becomes uncertain, and the two events are not independent. That correlation is invisible in any analysis that looks only at the brokerage account.

The same logic applies with more force if you own a business. Someone whose company represents the bulk of their net worth is running the most concentrated position available, and the public-market holdings are a rounding error against it. The first step there is not a portfolio adjustment at all, it is getting an independent read on what the business is actually worth, because a concentration you have never valued cannot be managed.

Vesting schedules also mean employer stock concentration rebuilds itself. Trimming once does nothing if new grants land every year. The fix is a standing rule, such as selling on a schedule at vest, rather than a one-time decision you will have to make again with worse information each time.

The S&P 500 has the same problem right now

This is the part that surprises people who assume an index fund solves concentration. A cap weighted S&P 500 fund's ten largest holdings were 36.33% of the fund in July 2026, against 2.50% for the ten largest holdings of an equal weighted version of the same 500 companies. Identical constituents, wildly different risk. Published measurements of top-10 index concentration have ranged from roughly 37% to 43% over 2025 and into 2026 depending on the measurement date and whether the figure is for the index or a specific fund, so always check the date attached to any number you see quoted. What is not in dispute is the direction: it is the highest on record, and it is driven by a handful of AI-linked megacaps.

The implication for position sizing is direct. If you hold an S&P 500 fund and separately own a large position in one of those same megacaps, your true exposure to that company is the sum of both, and the fund is quietly adding to a position you thought you had sized carefully. That look-through exposure is the single most common thing people miss when they check their weights, and it is why a portfolio analyzer that reads holdings as one combined weighted index is more useful than a screen showing accounts side by side.

How to check your own concentration in ten minutes

Three passes, in order.

Convert everything to weights. Dollars hide the shape. Write each position as a percentage of the total portfolio and sort descending. Most people find the answer to this article's question sitting at the top of that list, and no further analysis is needed to see the problem.

Add up the look-through exposure. If you hold funds as well as individual stocks, find the fund's top holdings and add your share of them to your direct positions in the same names. A 6% direct position in a megacap plus a 40% allocation to an index fund holding that company at 7% is really about 8.8%, not 6%.

Test what the concentration actually did. Build the basket as a weighted index, run it over real market history, then run it again with the large position capped at 5% and the difference spread across the rest. Comparing the two curves shows you exactly what the concentration bought and what it risked, in drawdown terms rather than in theory. You can backtest the portfolio both ways and read the results side by side, and a fitting benchmark tells you whether the outcome came from the concentration or from the market.

What to do if you are over the line

Trimming is the obvious answer and the tax bill is the obvious obstacle, which is why so many oversized positions stay oversized. A few approaches that do not require selling everything at once:

Stop reinvesting into it. Direct dividends and new contributions elsewhere. This does nothing immediately and quite a lot over three years, and it triggers no tax at all.

Trim on a schedule rather than on a view. Selling a fixed percentage quarterly until you reach your target removes the timing decision, which is the part people get wrong. Waiting for a good price to sell a position that is too large is how positions stay too large.

Use the losses you already have. If other holdings are underwater, realizing those losses can offset gains on the trim. Net capital losses beyond that offset up to $3,000 of ordinary income per year, $1,500 if married filing separately, with the excess carried forward, per IRS Topic 409. Watch the wash sale rule under IRC 1091 if you plan to buy back in.

Decide the target weight in advance and write it down. A rule set when you are calm survives a bull market. A judgment call made while a position is running does not. The mechanics of holding weights to a target are covered in portfolio rebalancing, and the weighting choices themselves in index construction.

The short version

Use 5% as the default ceiling for a single stock, 10% if you run a more concentrated book you actually follow, 10% to 15% maximum for employer stock, and treat anything past 20% as a position that needs a written plan rather than a hope. Then check your look-through exposure through any index funds you own, because that is where the concentration you did not choose is hiding. If you want to see how far apart equal weighting and cap weighting put two portfolios with identical holdings, running both is the fastest way to make it concrete.

Indexes is educational and informational software. It never places trades, connects to a brokerage or holds assets, and nothing here is investment advice or tax advice. Backtests are hypothetical and past results do not predict future returns.