How Much of Your Portfolio Should Be in a Single Stock?
The stock that’s gone up the most is always the hardest one to sell. Maybe it’s employer stock you’ve been granted for years, maybe it’s a position you bought early and never touched again — either way, it can quietly become a huge slice of your net worth while you keep telling yourself “just a little more upside.” That’s concentration risk: a portfolio whose outcome hinges on a single stock, sometimes called a concentrated position. There’s no universal rule that says “X% and above is dangerous.” But there are widely used industry benchmarks, well-documented reasons we keep crossing them, and — later in this piece — an actual calculation of how long it takes to unwind a concentrated position once you decide to.
One thing to clear up first: this article is about a stock you personally hold outright, including employer stock. It’s a different problem from the automatic concentration that builds up inside an index fund, where a handful of mega-cap names can quietly dominate the fund’s weighting. That’s a separate topic covered in Index Fund Concentration: How Risky Are the Top Holdings? — worth reading if that’s what brought you here.
What Concentration Risk Actually Is
Concentration risk is the “all your eggs in one basket” problem, made literal. When most of a portfolio sits in one company, one sector, or a handful of closely correlated assets, anything that happens to that one thing happens to nearly everything you own. As covered in Why Diversification Reduces Risk — and Its Limits, diversification works because you’re combining assets that don’t all move together, so one asset’s bad day gets cushioned by another’s ordinary day. A concentrated position is the opposite setup: there’s little or nothing left to absorb the hit.
It’s easy to assume “single stock” only means employer stock, but that’s too narrow. It also includes a stock that happened to run up years ago, an inherited position, or founder’s equity you never rebalanced away from. What they all share is the same root cause: the weight in your net worth wasn’t something you designed on purpose. It’s something that happened to you over time.
How Much Is Too Much? Industry Rules of Thumb
Short answer: there’s no statistically proven threshold, but industry convention treats 5-10% as generally acceptable, 10-15% as worth reconsidering, and 20-25%+ as high risk. These are conventions from decades of advisor experience, not a scientifically optimized breakpoint.
The question I get most is some version of “at what percentage does this become dangerous?” Honestly, there’s no statistically optimized answer here, no peer-reviewed formula that says “cross this line and your expected outcome drops by X%.” What exists instead are benchmarks that wealth managers and advisors have converged on through decades of client conversations. The table below reflects that convention. It is not a scientifically proven breakpoint.
| Position size | Common interpretation (industry convention, not a rule) |
|---|---|
| Under 5% | Low risk |
| 5-10% | Generally acceptable, worth monitoring |
| 10-15% | Worth reconsidering |
| 15-20% | Caution — many advisors flag this as a warning line |
| 20-25%+ | High risk — reduction typically recommended |
There’s a detail that changes how this table should feel to you: what’s the denominator? 20% of your investment portfolio is a very different situation from 20% of your total net worth, once you count home equity and cash. Say your investment portfolio is $30,000 and 20% of that ($6,000) sits in one stock. If your total net worth, including your home equity and savings, is $100,000, that same $6,000 is only 6% of everything you actually own. If your investment portfolio is essentially your entire net worth, those two numbers converge instead.
I’ve talked to plenty of people who heard “20% is risky” and repeated it back to me without ever actually running the math on their full net worth. The benchmark is a starting point, not a verdict. Work out what actually happens to your life if that position drops hard before you decide whether the number on the table applies to you.
Why We End Up Overweight One Stock — Behavioral Biases
Almost everyone already knows the principle: don’t bet everything on one stock. Yet positions still keep growing past the point most people would call comfortable. A few well-documented psychological patterns explain why.
The first is anchoring. Your purchase price becomes a fixed reference point in your head, even though the market has no idea what you paid for it. If the stock is below that price, you wait to “at least get back to even.” If it’s above, you wait for “a little more upside.” Either way, the anchor keeps pushing the decision into the future.
Layered on top of that is the disposition effect, a pattern researchers have documented for decades. Shefrin and Statman (1985) formalized it in their Journal of Finance paper, and Odean’s (1998) analysis of actual trading records confirmed the same behavior in real accounts: investors tend to sell winners too early and hold losers too long, reluctant to lock in a loss. With a big concentrated winner, these two forces oddly combine — “I should probably trim this” fights with “I’ve held it this long, it feels wasteful to sell now” — and inertia usually wins.
Then there’s the tax question. In my experience, the sense that “selling triggers a tax bill” is one of the strongest reasons people delay a decision they otherwise know they should make. Tax rules vary enormously by country and account type, so this article won’t get into specific strategies. Talk to a tax professional before unwinding a large position.
Finally, familiarity and loyalty bias tend to show up too. A company you work at, or feel like you understand well, tends to feel safer than it statistically is. Feeling like you know a company and actually being diversified are two completely different things, and it’s worth keeping them separate in your head.
What Concentration Risk Actually Costs You
“But my stock is different” is a hard bet to win, statistically speaking. Bessembinder (2018), published in the Journal of Financial Economics, tracked every one of the 25,967 stocks listed in the US between 1926 and 2016. The distribution of outcomes is lopsided almost beyond intuition: essentially all of the net wealth the stock market created over that period came from just the top 4.3% of stocks, 1,092 companies. The rest, in aggregate, added close to nothing. It gets starker. 57% of all listed stocks had lifetime returns lower than one-month Treasury bills. And of the 9,187 stocks that were delisted over that period, the median lifetime return was -91.95%.
The takeaway isn’t that stocks are bad. It’s that the odds of any single stock you happen to own being one of that winning 4.3% are lower than intuition suggests. Spread across many holdings, a small number of big winners statistically carry the group’s return — that’s a normal, expected pattern. Put everything into one stock, and you’re essentially buying a single ticket to that lottery.
Employer stock adds a second layer that’s specific to it: double exposure. In one well-documented case, a major public company’s accounting fraud scandal wiped out a stock that had accounted for more than 60% of employees’ retirement savings — the share price collapsed to essentially zero within a year (source). Those employees had their paycheck and their retirement nest egg tied to the fortunes of exactly one company. When the company fails, both fail at the same time. That’s what makes employer stock concentration a distinct, sharper version of this risk.
How Much Is “Too Much” for You — A Situational Checklist
The same 20% means different things to different people. Answering these five questions gives you a far more useful read than the table alone.
1. Is this employer-granted stock? A position you chose to buy is different from one that arrives automatically as part of your compensation. The latter builds up without you making an active decision each time, and if it’s your own employer, the double-exposure problem from the last section applies directly.
2. Do you need access to a large sum within the next five years? A down payment, tuition, or an approaching retirement date all mean a single stock’s volatility can directly disrupt real-life plans. The longer your time horizon, the more room you have to ride out swings; the shorter it is, the less room you have.
3. Is the current size mostly from a recent run-up, or built up gradually over years? A stock that spiked in the last few months may be carrying gains that haven’t been tested by time. A position that grew slowly over years is a somewhat different situation.
4. Have you actually calculated the percentage against your total net worth? As covered above, “20% of my portfolio” and “20% of my net worth” aren’t the same number. Until you actually run the calculation, most people are just guessing based on how it feels. I’ve watched people be surprised both ways: some find the number is lower than they feared, others find it’s higher.
5. Is “I don’t want to pay the tax” the only reason you’re not selling? If so, that’s a tax question, not an investing question, and the two deserve to be evaluated separately. Get a clear answer from a tax professional on what selling would actually cost, then make the investing decision on its own merits.
Once you’ve answered these five questions, you’ll have a much sharper picture than any single number on a chart can give you. For a broader framework on gauging how much risk you can actually tolerate, see How to Evaluate Investment Risk.
When and How to Reduce a Concentrated Position
Once you decide to trim a position, the next common mistake is trying to sell it all at once. Dumping a large position in one go concentrates your tax bill into a single year and adds a psychological trap on top: the nagging fear that you’re selling right before the peak. In practice, three principles work better together.
First, set your target weight in advance, not after the fact. A rule like “if this position ever exceeds X% of my portfolio, I trim it back” removes emotion from the exact moment emotion is most likely to take over.
Second, sell gradually, in tranches. Selling a fixed portion on a schedule — annually or quarterly — reduces how much any single day’s price determines your outcome. How long does that actually take in practice? I ran the numbers.
How Long It Takes to Cut a Concentrated Position Down to Target (Assumption: starting weight of 30%, sale proceeds reinvested elsewhere, and the two asset groups’ price movements are assumed to offset each other so we can isolate the dilution effect of selling itself. This is a simplified calculation that excludes taxes, fees, and any further price appreciation.)
| Annual sale rate (of remaining position) | Time to go from 30% to 15% | Time to go from 30% to 10% |
|---|---|---|
| 5% | ~13.5 years | ~21.4 years |
| 10% | ~6.6 years | ~10.4 years |
| 15% | ~4.3 years | ~6.8 years |
| 20% | ~3.1 years | ~4.9 years |
| 25% | ~2.4 years | ~3.8 years |
The math behind it is straightforward: if you sell a fixed share of what’s left each year, the position shrinks to (1 − sale rate)^t of its starting size after t years. The table just solves that equation for t at each target weight. Even a fairly moderate 10% annual sale rate takes more than six years to bring a 30% position down to 15%. If your plan is “sell it down slowly,” it’s worth knowing upfront exactly how long “slowly” really means — that’s the only way to actually stick with the plan. Keep in mind this table strips out price movement on purpose; in real life, the position’s actual value keeps changing while you’re selling, so your real timeline could run shorter or longer than this.
Third, put your review date on the calendar. Checking your concentration once a year, on a date you won’t forget — a birthday, year-end — works far better than a vague intention to “get to it eventually.”
One more thing worth saying plainly: trimming isn’t always the right call. If your time horizon is genuinely long, the position is a tolerable share of your total net worth, and your conviction in the stock rests on solid reasoning, there’s no rule that says you must sell. The real question to keep asking yourself is whether you’re holding it because you’ve actually run the numbers and can tolerate the risk, or simply because selling feels like a waste. For how this position fits into your broader allocation, see Asset Allocation Basics, and for what to reinvest the proceeds into, The Three-Fund Portfolio Strategy is a good place to start.
Frequently Asked Questions
What percentage in a single stock is considered safe?
There’s no statistically proven answer. Wealth managers commonly treat under 5% as low risk, 5-10% as acceptable but worth watching, 10-15% as worth reconsidering, 15-20% as a caution zone many advisors flag as a warning line, and 20-25%+ as high risk that usually warrants trimming. These are industry conventions, not guarantees, and whether you measure against your investment portfolio or your total net worth changes how the number feels.
Why is employer stock especially risky?
Because it creates double exposure: your paycheck and your retirement savings both depend on the same company. In one documented case, a major public company’s accounting fraud wiped out a stock that made up more than 60% of employees’ retirement savings, and the price collapsed to essentially zero within a year. Employees lost their jobs and their nest eggs at the same time.
What’s the practical difference between a 20% and a 50% position?
Simple arithmetic: if the stock drops 50%, a 20% position costs you 10 percentage points of your portfolio, while a 50% position costs you 25 points, a 2.5x difference. By industry convention, 20% already sits in high-risk territory, while 50% means most of your financial future is tied to one company’s fate, a fundamentally different kind of risk.
A stock I own has gone up a lot recently. Why is that a warning sign?
A sharp recent run-up can mean the gain hasn’t been tested by time, and the increase itself is what’s mechanically inflating your position’s weight. Unlike a position built up gradually over years, a recent spike often means the size wasn’t something you chose. The market chose it for you.
Does concentration risk exist inside index funds too?
Yes, but for a different reason than what this article covers. Market-cap-weighted index funds structurally concentrate weight into a handful of mega-cap stocks automatically. We cover that separately in Index Fund Concentration: How Risky Are the Top Holdings?
Is reducing a concentrated position always the right move?
No. If your time horizon is long, the position is a manageable share of your total net worth, and you have solid reasons to hold, there’s no rule saying you have to sell. What matters is checking whether that decision is based on actually running the numbers, not just on not wanting to sell.
Key Takeaways
- The right question isn’t “what percentage” but “can I actually tolerate it if this position drops hard, given my situation.” Industry benchmarks (15-20% as caution, 20-25%+ as high risk) are a starting point, not a verdict.
- Calculate your weight against both your investment portfolio and your total net worth separately. The same 20% feels very different depending on the denominator.
- Just recognizing anchoring, the disposition effect, and tax-driven inertia in yourself changes how you make the decision.
- Bessembinder’s (2018) data shows most individual stocks underperform. The odds that your one pick is among the small group of big winners are lower than they feel.
- If it’s employer stock, check the double-exposure problem first: your paycheck and your retirement savings shouldn’t both depend on one company.
- If you decide to trim, set a target weight in advance, sell in tranches, and put a review date on your calendar. But remember, trimming isn’t automatically the right answer for everyone.
If taxes are part of the picture — and for most people sitting on a large concentrated gain, they are — layer a tax professional’s advice on top of everything here before you make a final call.