Understanding Concentration Risk in Angel Investing with the HHI Score
Concentration risk is the silent portfolio killer. It builds gradually as one or two investments outperform, quietly shifting your portfolio's weight until a single company represents 40 or 50 percent of your total value. At that point, your diversified portfolio has effectively become a single bet, and you may not even realize it.
The Herfindahl-Hirschman Index (HHI) provides a quantitative measure of this concentration. Originally developed for antitrust analysis of market competition, HHI is equally useful for measuring how concentrated an investment portfolio is. For angel investors, tracking HHI is one of the most practical ways to monitor and manage concentration risk.
What Is the HHI Score?
The HHI is calculated by squaring the percentage weight of each investment in your portfolio and summing the results.
Formula: HHI = sum of (weight of each investment)^2
Where each weight is expressed as a percentage of total portfolio value.
Example: A portfolio with 5 equal investments (20 percent each): HHI = 20^2 + 20^2 + 20^2 + 20^2 + 20^2 = 2,000
Example: A portfolio where one investment is 60 percent and four others are 10 percent each: HHI = 60^2 + 10^2 + 10^2 + 10^2 + 10^2 = 4,000
The higher the HHI, the more concentrated the portfolio. The scale ranges from near 0 (maximum diversification) to 10,000 (a single investment).
HHI Thresholds for Angel Portfolios
Below 1,500: Healthy Diversification
Your portfolio weight is spread across multiple investments without excessive dependence on any single position. This is the target zone for most angel portfolios.
At this level, the failure of any single investment has a manageable impact on total portfolio value. You have enough diversification to weather individual company setbacks while still maintaining meaningful exposure to your winners.
1,500 to 2,500: Moderate Concentration
Your portfolio is beginning to concentrate, typically because one or two investments have appreciated significantly. This is not an emergency, but it warrants monitoring and consideration of rebalancing options.
Most angel portfolios naturally drift into this range over time as winner investments grow in value. The key question is whether the concentration is increasing or stable.
Above 2,500: High Concentration
Your portfolio has dangerous concentration. A significant portion of your value depends on the outcome of one or two investments. While this can produce spectacular returns if those investments continue to perform, it also means that a single adverse event could dramatically reduce your portfolio value.
At this level, you should actively evaluate rebalancing strategies.
How Concentration Builds Silently
The insidious nature of concentration risk is that it builds through success, not through mistakes. Here is how it typically happens:
Year 1: You invest $25,000 each in 10 companies. HHI = 1,000 (well diversified).
Year 3: One company's valuation increases 10x while others grow modestly. That investment now represents 35 percent of portfolio value. HHI jumps to approximately 1,800.
Year 5: The same company continues growing. It now represents 55 percent of portfolio value. HHI exceeds 3,500.
At no point did you make a concentration decision. You simply held your winners, which is generally good practice. But the portfolio has transformed from diversified to dangerously concentrated through organic appreciation.
This is why tracking HHI over time, not just at the point of investment, is essential.
Beyond Position Concentration
HHI measures concentration across individual investments, but angel portfolios face concentration risk across multiple dimensions.
Sector Concentration
Calculate HHI using sector weights instead of position weights. If 60 percent of your portfolio value is in SaaS companies, your sector HHI reflects significant concentration that would not be apparent from position-level HHI alone.
Stage Concentration
A portfolio heavily weighted toward pre-seed investments faces different systemic risks than one spread across stages. Calculate stage-level HHI to understand this dimension.
Vintage Year Concentration
If most of your capital was deployed in a single year, your portfolio is sensitive to the market conditions of that vintage. Vintage year HHI helps quantify this exposure.
Geographic Concentration
For investors with international portfolios, geographic concentration creates currency risk, regulatory risk, and macroeconomic correlation.
Platforms like AngelHub calculate concentration metrics across multiple dimensions automatically, surfacing risks that would be tedious to compute manually.
Strategies for Managing Concentration
Strategy 1: Controlled New Investment Targeting
When your HHI is elevated due to a single large position, deliberately target new investments in underweight sectors, stages, or geographies. Each new investment at your base check size gradually dilutes the concentration.
This is the simplest and most natural rebalancing approach. It does not require selling anything. It simply directs new capital toward areas that improve diversification.
Strategy 2: Secondary Market Sales
For larger positions in later-stage companies, secondary markets (Forge, EquityZen, or broker-facilitated transactions) can provide partial liquidity. Selling 20 to 30 percent of a concentrated position meaningfully reduces HHI while preserving upside exposure.
Secondary sales typically execute at a 10 to 30 percent discount to the most recent primary round valuation, which is a real cost. Evaluate whether the concentration risk reduction justifies the discount.
Strategy 3: Follow-On Capital Allocation
When deciding where to deploy follow-on capital, factor in concentration impact. Investing more in an already-concentrated position increases HHI further. Deploying follow-on capital into less concentrated positions may offer better portfolio-level risk-adjusted returns.
Strategy 4: Accept and Monitor
Sometimes, concentration in a high-quality investment is the right choice. If your most concentrated position is a company with exceptional fundamentals, strong growth, and a clear path to liquidity, the expected value of holding may exceed the risk reduction benefit of selling.
In this case, accept the concentration but monitor it actively. Set a threshold (such as HHI above 3,500 or a single position above 50 percent) that triggers a mandatory reassessment.
Common Questions About Concentration
Is Some Concentration Good?
Yes. A perfectly diversified portfolio with zero concentration means no investment has generated significant returns, which is a bad outcome. Some concentration is the natural result of winning investments growing in value. The goal is to manage concentration within acceptable bounds, not eliminate it.
How Does Portfolio Size Affect HHI?
Larger portfolios naturally have lower HHI. A portfolio of 30 equal-weight investments has an HHI of approximately 333. A portfolio of 10 equal-weight investments has an HHI of 1,000. More investments provide structural diversification that reduces concentration risk.
Should I Sell My Winners to Reduce Concentration?
Not automatically. The decision to sell should consider the company's prospects, available secondary market pricing, tax implications, and your overall portfolio strategy. Reducing concentration is one factor, not the only factor.
Conclusion
Concentration risk is the most common portfolio-level risk for angel investors, and it builds through success rather than failure. The HHI score provides a simple, quantitative way to monitor concentration across your portfolio and across multiple dimensions. By tracking HHI regularly and implementing deliberate rebalancing strategies, angel investors can capture the benefits of their winners while managing the risk that comes with portfolio concentration.
Frequently Asked Questions
How often should I calculate my portfolio's HHI score?
Monthly or quarterly is sufficient. HHI changes gradually as valuations update and new investments are made. More frequent calculation creates noise without actionable insight.
What is the ideal HHI for an angel portfolio?
Target an HHI below 1,500 for healthy diversification. Portfolios in the 1,000 to 1,500 range balance meaningful position sizes with adequate risk spreading.
Can I have a low HHI but still face concentration risk?
Yes. If all your investments are in the same sector or were made in the same year, your position-level HHI may be low while your sector-level or vintage-level HHI is high. This is why measuring concentration across multiple dimensions is important.
Does HHI matter for very small portfolios?
HHI is less informative for portfolios of fewer than 5 investments because concentration is inherent at that size. As your portfolio grows beyond 10 investments, HHI becomes increasingly useful for monitoring concentration trends.