Portfolio Concentration Risk Calculator

Concentration risk lives in the top holding. A 200,000 portfolio with a 60,000 single stock is 30% concentrated — one stock's bad year can drop the whole portfolio 10-15%. Most advisors flag concentration above 20% and serious advisors above 10%. Company shares from employer grants are a common concentration source — tax-efficient to hold but carry the concentration risk.

A worked example

Try the defaults: portfolio total of 200,000, largest single holding of 60,000. The tool returns 30.00%. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.

Now adjust the largest holding down to 40,000 while keeping the portfolio total at 200,000. The concentration falls to 20.00%. If you reduce it further to 20,000, concentration drops to 10.00%. This illustrates how even modest shifts in a single position reshape the diversification picture across an entire portfolio.

What moves the number most

The result responds to Portfolio Total and Largest Single Holding. The rate and the time horizon usually dominate — compounding means a small change in either reshapes the final figure more than a similar shift in contribution size. Test this by doubling one input at a time.

The formula behind this

Largest holding value divided by total portfolio value. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.

Using this well

Common scenarios where concentration matters

• Evaluating holdings from employee share schemes or long-held positions that have grown significantly
• Assessing the impact of a single stock or fund on overall portfolio risk
• Planning rebalancing or diversification across multiple asset classes
• Comparing concentration levels across different portfolio snapshots over time
• Understanding exposure when one investment dominates your holdings

What this captures

The calculator estimates what percentage of your total portfolio value is held in your single largest position. It quantifies concentration in isolation — the raw size of your top holding relative to the whole. This forms a straightforward snapshot of diversification exposure at a point in time.

What this doesn't capture

Concentration risk exists in context. The calculator does not account for correlation between holdings — two large positions in different asset classes behave differently from two similar positions. It does not model volatility, drawdown scenarios, or how concentration interacts with overall portfolio composition. Sector or geographic clustering may amplify concentration risk beyond what the headline percentage shows. The figure represents a static measurement rather than a forecast, and does not reflect how holdings may shift as values change over time.

This calculator is for educational illustration of diversification concepts. It does not constitute investment guidance.