What it means
A portfolio can hold several stocks yet still depend heavily on one company or one source of risk. A large position creates single-stock concentration. Smaller positions can also create concentration if they share a sector, country, currency, or other exposure and tend to react to the same event.
Concentration risk is about the consequences of those exposures, not just their size. Portfolio concentration describes how holdings are distributed; concentration risk asks how much a shared setback could affect the portfolio. An equal-weight basket may be less concentrated by stock than a basket dominated by one holding, but it can still be concentrated in one sector.
How it is calculated
There is no single formula that measures every form of concentration risk. Start with portfolio weights, then calculate the Herfindahl-Hirschman index (HHI) for a defined set of nonoverlapping holdings or exposure groups:
w_i = V_i / V_total—w_iis holding or groupi's weight, expressed as a decimal;V_iis its value in US$;V_totalis the total value in US$ of all holdings or groups included.HHI = Σ(w_i²)— square each decimal weight and add the results. For nonnegative weights summing to 1, HHI ranges from1/NforNequal-weight holdings to1for a single holding.
State what the weights represent. Stock-level HHI measures concentration across stocks; sector-level HHI uses sector weights and answers a different question. HHI is a concentration measure, not an estimated loss or a probability of loss. For baskets containing both long and short positions, the choice of net versus gross exposure also affects weights; a simple long-only HHI should not be applied without specifying that choice.
Worked example
Suppose a hypothetical, long-only US$100,000 stock portfolio has four positions:
| Stock | Value | Weight | Weight squared |
|---|---|---|---|
| A | US$50,000 | 50% | 0.25 |
| B | US$20,000 | 20% | 0.04 |
| C | US$20,000 | 20% | 0.04 |
| D | US$10,000 | 10% | 0.01 |
First, divide each position value by US$100,000. Stock A's weight is US$50,000 / US$100,000 = 0.50. Next, square the decimal weights and add them: HHI = 0.25 + 0.04 + 0.04 + 0.01 = 0.34. Four equally weighted stocks would have HHI = 4 × 0.25² = 0.25, so this basket is more concentrated by stock.
If A hypothetically falls 20% while the other prices do not change, its US$50,000 position loses US$10,000. That is 10% of the portfolio's starting value. This illustration is not a forecast; it isolates one holding's contribution and excludes trading costs and other price changes.
Why it matters before you trade
A proposed trade can change the basket's largest weight and its exposure to a sector, country, or currency. Checking these before placing an order makes the resulting portfolio easier to understand. Consider both the proposed basket and any existing holdings: a trade that looks small on its own may add to an exposure already present.
Position weights and HHI do not capture every risk. Two different stocks may respond similarly to the same news, while a single holding may have risks not shared by others. Correlation, volatility, and the ability to exit a position are separate considerations. A concentration check identifies where exposure sits; it does not establish a universally safe weight.
How PreTrAIde uses it
PreTrAIde provides pre-trade analysis for stock baskets before users place any order. Its quantitative layer calculates portfolio breakdowns by country, sector, region, currency, and market capitalization, as well as the Herfindahl-Hirschman concentration index. Its free portfolio concentration checker reports HHI and the largest weight. A basket analysis returns a GO, NO-GO, or MODIFY verdict with a risk assessment and plain-language reasoning; those verdicts are decision support, not recommendations. Users place trades themselves through their own broker.
Common mistakes
- Counting tickers instead of weights: Four holdings are not necessarily four equal exposures.
- Treating HHI as a loss estimate: It summarizes weight distribution, not how far prices might move.
- Ignoring shared exposures: Different companies can still depend on the same sector, country, or currency.
- Checking only the new basket: Existing holdings may change the combined exposure substantially.
- Mixing weight definitions: Net weights, gross weights, and long-only weights can produce different readings, especially with shorts.