There is no official limit — but investor-education sources converge on the same ranges: under 5% of a portfolio in one stock is generally treated as well diversified, 5–10% as a high-conviction position to be monitored, and above 10% as the point where one company’s news can dominate results. Above 20% is widely described as over-concentration. The figures are conventions, not rules — and they apply to total wealth, employer stock and the shares inside your funds included.
Most concentration is unplanned. A position bought years ago compounds faster than everything around it; an employer vests shares into an account every quarter and nobody sells; three “diversified” funds turn out to own the same seven companies. The result is the same: one name at 25% or 30% of everything, arrived at without a single decision. This guide covers the thresholds investors actually use, how to measure exposure properly — across accounts, not within one — and what the arithmetic says when that name has a bad year. Avex provides mathematical context, not investment advice.
Key Takeaways
- 5% / 10% / 20% are the thresholds investor-education sources cite most; the 10% line is the one most risk tools flag.[1][2]
- Exposure is measured against total wealth, not one account: vested employer shares, an ESPP, a 401(k) company-stock fund and a brokerage holding in the same name are a single position.
- Index funds hide overlap: the largest companies in the S&P 500 are the largest in most growth funds too, so a “diversified” portfolio often carries a single-name concentration nobody chose.[3]
- Concentration is a drawdown problem, not a return problem: a 30% position that halves takes 15% off the whole portfolio, whatever the rest does.
Where do the 5%, 10% and 20% thresholds come from?
From risk-management practice — not from any regulator. T. Rowe Price treats a single stock above 5% as worth addressing, “particularly when it involves company stock”, and above 10% as warranting more immediate planning.[1] Fidelity’s investor education frames the same question — do you hold too much in one investment? — and sets out why a large single holding deserves review.[2] Across advisers and personal-finance publishers the three bands recur with minor variation: roughly 5% as conservative, 10% as the line where one company starts to drive results, 20% or more as over-concentration. The edges differ; the structure does not.
Two properties make the bands useful rather than arbitrary. First, they describe single-company risk, not market risk: an earnings miss, a lawsuit, a regulatory finding or a leadership exit can move one stock 30% in a day in a way a diversified index rarely moves. Second, they scale linearly, which is easy to underestimate until it’s your own number. A 10% position that halves costs the portfolio 5%. A 30% position that halves costs 15% — the kind of loss that changes plans, not just moods.
How do you measure your real exposure to one stock?
Add every form of the same company across every account, then divide by total investable wealth. Most investors stop at the brokerage app — the screen where the number looks smallest. The complete count includes:
- Shares held in taxable accounts, IRAs and old 401(k)s, at every broker.
- Vested RSUs and ESPP shares — they arrive automatically and are easily forgotten.
- Unvested grants, counted separately but counted: exposure that cannot yet be sold.
- The company-stock fund inside a 401(k), where the employer offers one.
- The slice of the same company inside index and growth funds (next section).
Our portfolio diversification calculator handles the first four: paste holdings and weights and it returns the largest position, the top-3 share and an effective number of holdings. The fifth requires holdings data for each fund — a tracker’s job, not a calculator’s.
Why does an index fund not fix this?
Because the same few companies sit at the top of nearly every broad US fund. The S&P 500 is weighted by market value, so its largest constituents carry a large share of the index; S&P Dow Jones Indices publishes the current top-ten weight on the index factsheet.[3] A growth fund, a technology fund and a “quality” fund typically hold the same names at higher weights still. An investor holding an S&P 500 fund, a Nasdaq-100 fund and a handful of individual technology shares can reach 15% or more in one company without ever buying it deliberately. This look-through exposure is invisible inside any single app — it appears only when every holding is on one screen, which is the screen most investors have never built.
What does a concentrated position do in a downturn?
It converts the company’s drawdown into the portfolio’s drawdown, in proportion to its weight. The arithmetic is unforgiving, and worth running once with your own figures:
A $400,000 portfolio holds $120,000 (30%) in one stock. The stock falls 50% while the rest of the portfolio is flat. The portfolio is now $340,000 — down 15%. Recovery requires the stock to double, or the remaining 70% to gain roughly 21%. The same 50% fall on a 5% position costs 2.5%: a bad week, not a bad year. Illustrative figures; no particular company or outcome is implied.
For historical-style falls and recovery times rather than a round number, the drawdown simulator runs the same arithmetic on your own weights.
What do investors commonly do about it?
The options are well documented; which one fits is a personal and usually a tax question — and this guide does not answer it. Investor-education sources discuss trimming gradually rather than at once, directing new contributions elsewhere so the position shrinks as a share without a sale, using tax-advantaged accounts where a sale carries no immediate tax cost, and — for very large single-stock positions — structures that a licensed adviser would need to explain.[1][2] A sale in a taxable account is a capital gains event; the IRS summarises the rules in Topic 409.[4] What every option shares is a starting point: a measured number. That is the only part of the problem software solves — and the part most investors skip.
Avex AI provides software and educational content: mathematical context, never investment or tax advice. The thresholds on this page are common analytical conventions, not recommendations, and nothing here suggests buying or selling any security. Capital at risk.
Ten percent is the line most risk-management guidelines treat as the point where a single company can dominate results — and the one most tools flag. Whether it is too much depends on what the stock is (an employer’s shares carry income risk as well), what else is held, and whether the position was chosen or simply grew.
Education sources rarely give a single figure. The usual framing is that individual stocks are the part of a portfolio an investor could watch fall sharply without changing plans, with broad funds doing the heavy lifting. The per-stock bands (5 / 10 / 20%) are the better-defined convention.
Yes — and it counts twice: as a holding and as the source of income. Vested RSUs, ESPP shares and a 401(k) company-stock fund are the same bet as shares bought on the open market, and investor-education sources single company stock out as the position most worth addressing early.
Export the holdings from each account, combine them, and compute each company’s weight against the total. The diversification calculator on this site does the arithmetic from a pasted list; a dedicated tracker does it from statements, including the look-through into funds.
One whose results are driven by a small number of positions rather than by the market. A practical test is the effective number of holdings: twenty lines where one is 60% of the total behave like fewer than three equal-sized positions.
Sources & references
- T. Rowe Price: Actions you can take if your portfolio is too concentrated in one equity
- Fidelity Learning Center: Do you hold too much in one investment?
- S&P Dow Jones Indices: S&P 500 (factsheet with current top-ten constituent weight)
- IRS: Topic No. 409, Capital Gains and Losses
Last verified: 22 October 2026.
