Portfolio Concentration Risk: What It Actually Costs You When One Stock Is 40%
5 min read
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5 min read
Check your holdings right now. If one stock has run up a lot, there is a good chance it has quietly grown into 40% or more of your entire portfolio, even if you never meant to bet that big on a single company.
That is portfolio concentration risk: the amount of your money whose fate now depends on one company's decisions, one earnings report, one piece of news. It does not show up as a warning anywhere. It just sits there, growing every time the stock has a good month, until one bad day shows you exactly what it was costing you.
Concentration risk is simply how much of your outcome rides on one holding instead of being spread across many. A portfolio where no position is above 10% behaves very differently from one where a single stock is 40%, even if both portfolios hold "good" companies.
The mechanism is straightforward: if a position is 40% of your money and it falls 20%, your whole portfolio takes an 8 percentage point hit from that one stock alone (0.40 × 20%). If the same stock were 10% of your money, the identical 20% drop only costs your portfolio 2 percentage points. Same company, same bad day, four times the damage, purely because of the weight you gave it.
Nvidia is a useful example precisely because so many investors let it run into an oversized position without deciding to. It had been one of the best-performing large stocks for two years straight, and a stock that keeps winning quietly eats a bigger and bigger share of a portfolio all on its own, even if you never buy another share.
On 27 January 2025, Nvidia fell 16.9% in a single session after the Chinese AI lab DeepSeek released a model that appeared to match top Western AI systems at a fraction of the training cost, raising doubts about how much AI chip spending would be needed going forward. The stock lost close to $589 billion in market value that day, the largest single-day market cap loss for any company in history (CNBC, Forbes). For context, the S&P 500 fell about 1.5% and the Nasdaq Composite fell about 3% that same day, so this was not just a bad day for the market in general, it was a specific, oversized hit to one company.
Here is what that single day cost a Rp 100,000,000 portfolio, depending only on how much of it was sitting in that one stock. This isolates the effect of the position's weight, holding the rest of the portfolio unchanged for comparison:
| Weight in the one stock | Portfolio-level loss that day | On a Rp 100,000,000 portfolio |
|---|---|---|
| 40% | −6.76% | −Rp 6,760,000 |
| 20% | −3.38% | −Rp 3,380,000 |
| 10% | −1.69% | −Rp 1,690,000 |
The uncomfortable part is that you do not need to deliberately pick one stock and load up on it. Concentration builds on its own, simply because winners grow faster than everything else around them.
This is visible at the market level too, not just in individual portfolios. The so-called "Magnificent Seven" stocks (Apple, Microsoft, Nvidia, Alphabet, Amazon, Meta and Tesla) made up roughly 31.5% of the entire S&P 500's market value as of late July 2026, up from about 13% back in 2018 (Forbes, History of Market). Someone who bought a plain S&P 500 index fund years ago and never touched it now carries far more concentration in a handful of tech names than they signed up for, without making a single active decision.
The same drift happens inside a personal portfolio. Buy five stocks in roughly equal size, let one of them triple while the others move sideways, and it can end up as 40% of your money even though you never added to it again.
This is not a case for avoiding strong performers or trying to time an exit from anything. The point is narrower: know your own number. A position you would never deliberately build to 40% of your savings can get there anyway if you are not checking, and the first time you find out is usually the worst possible time, on a day like 27 January 2025.
Two checks are enough to catch it early:
Open your Holdings view in NetWort and look at what percentage of your portfolio each position actually represents today, not what it was when you bought it. If one line is well ahead of the rest, that is your concentration number, and now you know it instead of finding out on a day like Nvidia's $589 billion one. It pairs with the same lesson in why a volatile investment's average return and its actual result can tell two different stories: the risk that a single number can hide is exactly what shows up when a concentrated position has a bad day.