Position Size and a 40% Drop: What the Loss Really Costs
Take two accounts of $100,000 that each own the same stock, and let that stock fall 40 percent. One account had 5 percent in it and lost $2,000. The other had 30 percent and lost $12,000. Same company, same news, same chart, and a six-fold difference in damage.
Nothing about the stock explains that gap. Size does. I think most investors spend far too long on whether to buy and far too little on how much, and that is the argument of this post: the loss you can take from being wrong is set when you type the order quantity, not when the price falls.
The arithmetic behind one bad position
The loss to the whole portfolio equals the position’s weight multiplied by the size of the drop. There is nothing more to it. A 40 percent fall in a 5 percent position costs 2 percent of the account. The same fall in a 25 percent position costs 10 percent.
| Position weight | 20% drop | 40% drop | 60% drop |
|---|---|---|---|
| 2% | 0.4% | 0.8% | 1.2% |
| 5% | 1.0% | 2.0% | 3.0% |
| 10% | 2.0% | 4.0% | 6.0% |
| 15% | 3.0% | 6.0% | 9.0% |
| 25% | 5.0% | 10.0% | 15.0% |
| 30% | 6.0% | 12.0% | 18.0% |
Read across a row and you see how the damage grows with the drop. Read down a column and you see how it grows with the weight. The bottom-right corner is where accounts get hurt. An 18 percent portfolio loss from one holding is a bad year for a diversified fund, and here it comes from a single name.
Why the recovery is harder than the loss
Percentages are lopsided. A stock that falls 40 percent needs a gain of 66.7 percent to return to where it started. A stock that falls 50 percent needs 100 percent. At 60 percent the required gain is 150 percent.
| Drop | Gain needed to break even |
|---|---|
| 20% | 25.0% |
| 30% | 42.9% |
| 40% | 66.7% |
| 50% | 100.0% |
| 60% | 150.0% |
This is why the 30 percent position is worse than the 12 percent loss suggests. The stock has to rise two thirds just to give back what was lost, and while it does, the account is smaller and the recovery is compounding on a smaller base. Some stocks manage that. Plenty never do.
How often should you expect a 40 percent fall?
More often than the calm of a rising market suggests. Large, well-known companies have had years in which the share price lost 40 percent or more, usually when earnings disappointed at a moment when the multiple was high. I would not put a precise frequency on it, and anyone who does is guessing. For sizing, the useful assumption is simpler: any holding could do this while you own it.
That assumption changes how you read a stock like Tesla, where the argument for owning it and the argument for being careful can both be true at once. It also applies to a company with a good story and a demanding multiple. My earlier note on what a P/E of 35 requires makes the point from the other side: the higher the price of the story, the further it can fall when the story slips.
A cap you can write down
The rule I would use is a formula. Decide the loss you can accept from any single position being completely wrong, assume a drop size, and divide.
If you can live with a 2 percent hit to the portfolio and you assume a 40 percent fall is possible, the cap is 2 divided by 0.40, or 5 percent. If you can accept 3 percent and assume a 50 percent fall, the cap is 6 percent. If you are stricter, say 1 percent with a 40 percent fall, the cap is 2.5 percent.
Then write the cap next to the ticker. I am serious about that. A number in your head bends with your mood, and a number on the page does not.
The assumed drop should vary with the stock. For a steady, low-beta business I might assume 30 percent. For a young company, a cyclical, or something trading at a high multiple, 50 or 60 percent is fairer. My piece on sizing when beta is above 2 covers that side in more detail.
Winners quietly break the cap
Here is the trap that catches careful investors. You buy at 5 percent, the stock triples, and it is no longer 5 percent.
Say a $100,000 account holds $5,000 of one stock and $95,000 of everything else, and the everything-else stays flat. Triple the stock and the position is worth $15,000 in a $110,000 account, a weight of 13.6 percent. The 40 percent fall that once cost 2 percent now costs $6,000, or 5.5 percent of the account. Nothing in the analysis changed. Only the arithmetic did.
So a winner deserves a smaller weight than its gains suggest, and the cap has to be checked on the way up as well as the way down. Trimming a stock that has done well feels wrong, because the thesis is working. But the thesis and the weight are separate questions. A position can be right and still be too big.
The same logic says to be smaller in cyclicals and in stocks near their highs. A stock that has doubled from its low and trades at a rich multiple has more room to fall than one that is already near the bottom of its range. I would not treat those two as equal, even if my confidence in both is the same.
Averaging down raises the stakes
Adding to a falling stock raises your position size at the exact moment the stock is proving you wrong. It can be the right call, but only if you decide it with the same cap that governed the first purchase.
Work it through. An account of $100,000 buys 100 shares at $50, a $5,000 position at a 5 percent weight. The stock falls 40 percent to $30, so the position is $3,000 and the account is $98,000. Someone convinced the drop is an overreaction buys 100 more shares for another $3,000. Now there are 200 shares at an average cost of $40, and the position is $6,000 of a $98,000 account, about 6.1 percent, larger than the original cap.
If the stock then falls another 40 percent to $18, the 200 shares are worth $3,600 against $8,000 paid. The loss is $4,400, or 4.4 percent of the original account. The single position has cost more than double what the cap allowed, and the break-even sits at $40, a 33 percent gain from $30 but a 122 percent gain from $18.
Averaging down is not forbidden. The rule is that the second purchase must fit inside the cap on total cost, not just on today’s smaller market value. Under a 5 percent cap on cost, the account above would have had room for $5,000 total, and the second buy would have needed to be smaller or skipped.
Conviction is not a size
I can be certain about a thesis and still hold a small position, because being certain has never stopped a stock from falling. Certainty is a feeling about the company. Position size is a limit on your own error.
That separation is the hardest part in practice. The stocks people are most sure about are the ones they own the most of, and they are also the ones where a surprise hurts most, since nobody has prepared for it. The bull case for a company like SpaceX after its 53 percent drop can be well argued and the position can still be small. The two things are not in conflict.
Pairing the cap with a sell rule
A cap limits how much you own. A written sell rule limits how far a position can fall before you act. Together they put a ceiling on the damage from any one mistake. A 5 percent position with a rule to exit after a 25 percent decline costs at most 1.25 percent of the account, before slippage, and that arithmetic is what makes owning volatile stocks tolerable at all.
The rule has a cost. You will sell some stocks that then recover, and you will feel foolish. My view is that this is the price of not being wiped out by the ones that do not. It could be wrong in a specific case, and I accept that: if a stock drops on a market-wide panic that has nothing to do with the business, holding through can be right. The written rule should say what you will do in that case too, or you will improvise under stress.
Checking the number, in practice
Pick the single-position loss you can accept. Divide by the drop you think is possible. That is your ceiling. Write it next to the ticker.
Then check it at two moments: whenever you add to the position, and whenever the stock rises enough to change its weight. Quarterly is a sensible schedule for the second check. If any holding is above its cap, the fix is to trim it back or to accept, in writing, that you are running a larger risk than planned.
The number I would watch first is your largest position. Look at its weight today and multiply it by 40 percent. If the result is above the loss you said you could accept, the cap is already broken, and the next step is to cut the size until it fits. For the SEC’s plain-language guidance on risk and diversification, see the SEC investor resources education pages and the FINRA investor resources.
Analysis and opinion only, not investment advice. Figures come from simple arithmetic on round hypothetical numbers, not from any specific account or trade; general risk guidance comes from SEC investor resources and FINRA, and the calculations were checked on September 22, 2026.
Sources: Dividends (SEC Investor.gov glossary) (https://www.investor.gov/introduction-investing/investing-basics/glossary/dividend) · Dividends tax topic (IRS) (https://www.irs.gov/taxtopics/tc404)