There is no single safe percentage. There is a test that finds yours, and it starts by imagining the stock goes to 0.
Published: 17 August 2026 · By Spencer Li, CFTe
“How much of any one stock is safe to hold in a portfolio?” Someone asked exactly that on Reddit’s r/stocks, and it is one of the most common questions new investors ask.
The honest answer is that no single percentage is safe for everyone. The right size depends on how much of your whole portfolio you could afford to lose to one company.
The clean way to find it is what I call the Zero Test. Assume the stock goes to 0, and ask whether you could live with the hit.
A 5% position that goes to 0 costs you 5% of your portfolio. A 20% position costs you 20%, and a 50% position costs you half. Pick the largest loss you would accept from any single company, and that number is your ceiling.
Then keep checking it, because a winning stock quietly grows into a bigger share than you ever chose. Below are the numbers, why a stock going to 0 is not as rare as it feels, and the 2 other places concentration hides.
Why is there no single safe percentage?
Because 2 people holding the same stock at the same weight can be taking very different risks, depending on how they hold it, how long they plan to keep it and what else sits in the account beside it.
The first trades it with a stop-loss. Their risk is set by the distance to the stop, and the position size follows from that. The 2% money management rule covers that side of it.
The second holds it for years with no stop at all. Their risk is the whole position, because nothing gets them out on the way down.
What else they own matters too. A lot. 10% in one bank stock means something different when the other 90% is cash than when it is 4 more bank stocks.
So instead of a number, you need a test you can run on your own portfolio.
What does the Zero Test show?
The Zero Test asks one thing: if this stock fell hard, or went to 0, what would it cost the whole portfolio? The answer is the position’s weight multiplied by the fall.
| Position size | Stock falls 30% | Stock falls 50% | Stock falls 80% | Stock goes to 0 |
| 5% of portfolio | −1.5% | −2.5% | −4% | −5% |
| 10% of portfolio | −3% | −5% | −8% | −10% |
| 20% of portfolio | −6% | −10% | −16% | −20% |
| 50% of portfolio | −15% | −25% | −40% | −50% |
Read the last column first. That is the cost if you were wrong about the company in the worst way.
Then look at what it takes to earn it back. Losses and gains are not symmetrical:
| Position goes to 0 | Portfolio loss | Gain the rest needs to get back to even |
| 5% position | −5% | +5.3% |
| 10% position | −10% | +11.1% |
| 20% position | −20% | +25% |
| 50% position | −50% | +100% |
A 5% mistake needs a 5.3% gain on the rest to repair. A 50% mistake asks the rest of your portfolio to double just to get back to where you started.
There is no right row. The test is personal: find the row where the last column still lets you sleep, and that is your ceiling for any one name.
Is a stock going to 0 really that likely?
It feels rare, because the stocks we hear about are the survivors. The long-run record says otherwise.
Hendrik Bessembinder of Arizona State University studied the lifetime returns of every US common stock traded on the New York and American stock exchanges and the Nasdaq since 1926. He found that just 86 stocks accounted for “half of the stock market total” of wealth created “over the past 90 years”.
The other side of that finding is the part that matters here. In his words, “the remaining 96 percent of stocks collectively matched one-month T-bills.”
Do note the word “collectively”. It describes the group as a whole and says nothing about any single stock inside it. But it tells you how lopsided single-stock returns are. The market’s long-run gains came from a small handful of companies, and holding one stock is a bet that you picked one of them.
That is why the Zero Test is worth running even on a company you believe in. Conviction does not change the shape of the odds.
Does a stop-loss make a big position safe?
It helps a great deal, but it does not make the Zero Test irrelevant.
A stop caps the loss you plan for. It does not cap the loss you can get. A stop order turns into a market order when its price is touched, and the SEC’s investor bulletin warns that the fill “can deviate significantly from the stop price” when prices move fast.
The classic case is an overnight gap. Bad results come out after the close, the stock opens 40% lower the next morning, and the stop you placed 8% below your entry fills somewhere near −40%, 5 times the loss you planned for. Order Types Explained: Market, Limit, Stop and Stop-Limit walks through why.
So even with a stop, ask what a large gap would cost the whole portfolio at your current size. If that answer scares you, the position is too big for the stop to protect.
Sizing every position so a single loss cannot hurt the whole account is the core of risk management. Our free 15-Minute Swing Trader’s Checklist puts that check in front of you before every trade. Get the free checklist →
How does a position grow past your limit without you buying more?
By winning. This is the part of concentration most people never notice, because it feels like good news.
Say you start with 10% in one stock and 90% in everything else. Here is what happens to its share as it rises, if the rest of the portfolio stays flat:
| The stock… | Its share of your portfolio |
| stays flat | 10% |
| doubles | 18.2% |
| triples | 25% |
| rises 5 times | 35.7% |
You never bought another share. But a stock that rises 5 times now carries more than a third of your portfolio, and its Zero Test result has gone from 10% to almost 36%.
This is drift. The check is simple. Every few months, compare each position’s current weight against the ceiling you set.
When one has drifted past it, you have 2 honest choices. Trim it back to your ceiling. Or raise the ceiling on purpose, knowing what the new Zero Test result is.
The only wrong choice is not noticing. A position you sized at 10% and now hold at 36% is a decision you never actually made.
The reverse problem is adding to a stock as it falls, which pushes the weight up on the way down. That has its own post: Why You Should Never Average Down.
Where else does concentration hide?
The Zero Test works on one ticker at a time. Concentration also hides in 2 places a ticker-by-ticker check will miss.
Overlap. 5 stocks in the same industry can behave like 1 stock in a bad year for that industry, because one piece of news hits them all at once. A fund you hold may own the same company you already hold directly, which adds to your real exposure. Add those together before you run the test.
Your job. If you hold shares in the company you work for, your salary and your shares depend on the same business. If it gets into trouble, both can be hit at the same time. Count that position twice when you decide what you could live with.
Putting the Zero Test to work
Run 3 checks on your own portfolio, once now and then every few months:
- The Zero Test. For each position, multiply its weight by 100%. Is every result a loss you could live with?
- The drift check. Has any position grown past the ceiling you set when you bought it?
- The overlap check. Group the positions that would fall together on the same news, and run the Zero Test on each group as if it were one stock.
None of this tells you which stocks to own. It tells you how much damage any one of them can do. If you want the maths of how repeated losses compound into a wiped-out account, read Risk of Ruin: How to Work It Out.
The size you chose when you bought a stock is not the size you hold today. The Zero Test is how you find out the difference before the market does.
One-stock concentration FAQ
Is it safe to have 50% of my portfolio in one stock?
Run the Zero Test: if that stock went to 0, you would lose half your portfolio and need the rest to double to get back to even. Whether that is acceptable is your call, but it is worth making with those 2 numbers in front of you.
Does an index fund count as one stock?
No. An index fund holds many companies, so one company failing hits it only by that company’s weight in the index. But check whether a big holding in the fund is also a big direct holding of yours, because that is overlap.
Should I sell a winner that has grown too big?
That is a choice between trimming it back to your ceiling and raising your ceiling on purpose. Either can be reasonable. What matters is that you decide with the new Zero Test result in front of you.
How often should I check my position sizes?
Every few months, and after any large move in one of your holdings. Drift happens slowly and then all at once, so put the check on your calendar.
Is this the same as the 2% rule?
No. The 2% rule limits how much you risk per trade when you trade with a stop-loss. The Zero Test limits how much any one company can cost you if everything goes wrong, stop or no stop. Use both.
Go deeper
This post sits under our guide to money and risk management, and our guide to building an investment portfolio covers the bigger picture. If you would like the routine that fixes your stop and size before every trade, the free 15-Minute Swing Trader’s Checklist is the place to start.
About the author. Spencer Li is The 15-Minute Trader and the founder of Synapse Trading. He is a Certified Financial Technician (CFTe) with 15+ years of trading, run from 70+ countries. He holds a B.Acc and B.BM from SMU. Every trade he takes is public, dated, losses left in. He teaches rules-based swing trading in 15 minutes a day, one system for any market.
Educational content only, not personalised financial advice. Trading carries risk, including loss of capital. Past performance is not indicative of future results. Synapse Trading is not licensed or regulated by MAS.
